Well into the night of Sunday, Jan. 2, 2005, lt. Cmdr. Matthew Diaz sat alone at his desk in the headquarters of the American detention center at Guantánamo Bay, Cuba, consumed with a new project.

He often worked late. From the time Diaz enlisted in the Army as a 17-year-old high-school dropout, hard work had been his ticket. He had earned his college degree while serving as an artillery sergeant and then completed law school a semester early, driving a mail truck on the weekends. In 10 years as a Navy lawyer, his performance evaluations had been outstanding. As his six-month tour at Guantanamo neared its end, his stint as the deputy legal adviser there looked like more of the same.

But the task that absorbed Diaz that night in January was taking him down a different path. Sitting at a secure desktop computer, he printed out page after page of classified information, pulling each batch from the printer in case anyone wandered by. When he was done, Diaz had assembled a document 39 pages long. In tiny type, it listed names, prison serial numbers and other information for each of the 551 men who were then being held by the United States at Guantanamo Bay.

There was no question of the government’s desire to keep the information secret. Six months earlier, the Supreme Court rocked the Bush administration by upholding the Guantánamo prisoners’ right to challenge their detention in habeas corpus proceedings. But the administration fought on, taking the narrow view that while the detainees might have been granted their day in federal court, they still had no “legal rights” — and specifically no right to counsel. Pentagon officials said that they were withholding the prisoners’ names for their own safety. But keeping the names secret made it harder for volunteer lawyers to file petitions on the prisoners’ behalf and for critics to dispute official claims that virtually all the men were terrorists.

Diaz’s indignation at the government’s policies had been building since he arrived at Guantánamo. He did not doubt that there were dangerous men there. But he had come to believe that the Pentagon was misrepresenting how the detainees were treated and the threat some of them posed. As a lawyer, he found the recalcitrance of the White House indefensible. The Supreme Court had spoken. Why couldn’t the administration go before a judge and show why these men should be held indefinitely and without charge? “I feel like I’m on the wrong side,” he confided to a couple of the lawyers who were representing Guantánamo prisoners.

Now, Diaz knew he was crossing a line. For nearly two weeks after printing the list, he kept it locked inside the safe in his office. On another late night, he carefully trimmed the pages down to the size of large index cards. Then, on Jan. 14, the last night of his tour, he went back to the office one more time. While his colleagues were getting ready for his farewell dinner, he slipped the stack of paper inside a Valentine’s Day card he had bought at the base exchange. It was an odd touch. The card showed a cartoon puppy with long ears and bubble eyes and the greeting, “Hope Valentine’s Day is just your style.” Diaz would later say that he chose it because it was big enough to hold the list. He also hoped the lipstick-red envelope might pass unscrutinized through the Guantánamo post office.

The flaws in Diaz’s plan became apparent soon after the red envelope reached the New York offices of the Center for Constitutional Rights, a left-wing legal-advocacy group that counted itself among the most zealous opponents of the administration’s Guantanamo policy. Diaz had addressed the card to Barbara Olshansky, a lawyer at the center whom he had never met. Weeks earlier, she had written to the Pentagon official overseeing detention policy, Navy Secretary Gordon R. England, asking for the names of the detainees so attorneys could offer to represent them. Diaz, who had been copied on the draft response, knew that the administration never considered granting the request.

Olshansky, a 43-year-old woman with a sharp legal mind and unruly black hair, opened the valentine and suspected an elaborate joke. “I have a lot of very wise-ass friends,” she would say later. “Why would I believe that someone would send me something with the return address Guantánamo?” There was no official stamp on the paper, nothing marking it as secret. Olshansky thought the whole thing was weird, but in a way that was unnerving rather than funny.

The center was then suing the government on a range of sensitive issues: the U.S.A. Patriot Act, immigrants’ rights, Guantanamo. As news of the card spread, some older lawyers at the center had no trouble imagining a government trap. A few of the younger ones were even more suspicious. They began writing notes to one another when they needed to discuss the valentine, just in case the office was bugged. “Everyone was asking, ‘Who could have done this?’ ” Olshansky recalled.

It hardly occurred to the lawyers that someone inside the detention-camp headquarters might be trying to help them, Olshansky told me not long ago. For all the public debate about Guantánamo, there was little sign that members of the military were defying their superiors. Uniformed lawyers who had been assigned to defend some of the prisoners before military commissions had begun criticizing the rules for those tribunals, but that dissent was explicitly tolerated by the Pentagon. Some Muslim servicemen at Guantanamo, including an Army chaplain, Capt. James Yee, had been investigated on suspicion of disloyal conduct. But that episode and the others seemed to suggest more about the high-security atmosphere of the camp than it did about any internal opposition to how the prisoners were treated. The valentine was different: no one had taken the law into his own hands quite like this.

Olshansky agonized for weeks over what to do. The center’s president, Michael Ratner, initially suggested giving the papers to the press. But after some consultation with other lawyers, Olshansky called the chambers of the federal district judge who was hearing her Guantanamo suit. Olshansky told the judge’s clerk she had received some information that might be relevant to the case. Could she send it over for safekeeping? She said she never expected the court’s response: Olshansky was instructed to turn the material over to the Justice Department instead.

On March 15, 2005, a federal agent in a black overcoat flew to New York from Washington. He took a cab to the center’s offices in downtown Manhattan and kept it waiting while he went to retrieve the card and its contents. Once the F.B.I. began to investigate, it had little difficulty narrowing the list of possible suspects. Diaz had printed the document from his own computer, bought the valentine at the base exchange and left his fingerprints on the list. This past May, Matthew Diaz became the only United States serviceman to be convicted and imprisoned for an act of insubordination directed at the Bush administration’s detention policies.

Diaz volunteered to go to Guantánamo in early 2004, after a year and a half as the deputy legal officer at Naval Station Great Lakes, a training base north of Chicago. He hadn’t been enthusiastic about the Great Lakes assignment, but in his steady, low-key way he did very well there. His superior, Cmdr. Peter J. Straub, recommended Diaz for early promotion, describing him as “the consummate naval officer” and “a stellar leader of unquestionable integrity.” Six good months at Guantánamo would certainly help his chances of making commander, Straub told him.

With other legal officers being shipped off to Afghanistan and Iraq, Cuba sounded pretty good to Diaz: challenging, not dangerous and reasonably close to Jacksonville, Fla., where his 12-year-old daughter lived with his ex-wife. He did not question the military’s need to detain some suspected terrorists, he told me later. At the Army’s legal academy in Charlottesville, Va., where he earned a master’s degree in 2002, he had not been among those outraged by the Bush administration’s decision to set aside the Geneva Conventions in the fight against terrorism. “My takeaway from all of that,” he told me in one of a number of conversations I had with him before and during his confinement at the Navy brig in Charleston, S.C., “was that there was still an order from the Defense Department that they be treated humanely.”

But as Diaz prepared to deploy in the spring of 2004, questions about how the law applied at Guantánamo were coming to the fore. Diaz had known a couple of the military defense lawyers who were criticizing the rules for the tribunals there; he admired their audacity in speaking out. He was also struck by one of the briefs filed in Rasul v. Bush, the federal lawsuit that asserted the right of the Guantánamo prisoners to contest their detention in the court. Two authors of the brief were retired leaders of the Navy legal corps in which Diaz served. “To be sure,” they wrote, “this is a perilous time, as the president has stated. But that does not justify indefinite confinement without any type of hearing or judicial review.”

The Supreme Court decided the Rasul case on June 28, 2004, a week before Diaz arrived on Guantánamo. On his first full day on the base, he went along to watch guards notify the detainees about the new system of parole-type review boards that the military was hurriedly setting up in response to the ruling. Diaz was skeptical of the plan. “It just didn’t seem right that we were creating this new process that no one really ever heard of instead of finding a way to let them get into district court,” he said later.

Diaz had seen his share of prisons, both military and civilian. But he had never seen anything like the wire-mesh cages at Guantánamo. The prisoners looked more sad than fearsome, Diaz said. In Camp 4, where more-compliant detainees lived in barrackslike quarters, Diaz came upon an older prisoner shuffling along with a walker. “This is what I’d been told were the worst of the worst?” he recalled thinking. One detainee stuck out his hand as Diaz walked up. He took it without thinking, and the guards shot each other looks. “I thought, O.K., I shouldn’t do that.”

As the deputy staff judge advocate for Joint Task Force Guantánamo, Diaz had a ground-level view of the legal dramas that were unfolding in the camp. He delivered lawyers’ mail to the detainees. He dealt with the prosecutors and defense attorneys preparing for the military commission trials. With that spring’s Abu Ghraib scandal still fresh, Diaz was assigned to begin compiling a spreadsheet for internal use on the abuse allegations registered at Guantánamo. He was uneasy, he said, but hoped that he could make a positive impact. “I figured I could do my part to make sure things were done right,” he told me. “I felt that way about the military-justice system.”

Later during his first month at the detention center, Diaz’s boss was off the island when a call came in from the regional military command that oversees Guantánamo from Miami. The Justice Department was proposing rules to the federal courts for the civilian lawyers who wanted to visit detainees. Justice officials wanted the military to be able to listen in on meetings between the prisoners and their lawyers, and Diaz was told to work with intelligence officers to come up with an explanation of why such monitoring was necessary.

Diaz said he went to report the assignment to the Guantánamo chief of staff, Col. Tim Lynch. It turned out that Lynch had been over this ground before. As Diaz sat down in his office, he recalled, Lynch dialed his counterpart in Miami and demanded to know why Washington was insisting on monitoring that the intelligence officers at Guantánamo had already said was unnecessary. “ ‘Why are we doing this?’ ” Diaz quoted Lynch as saying. “ ‘My guys have told me they don’t need it. The boots-on-the-ground guys, they don’t need it!’ ”

Lynch was irate, Diaz said. But Diaz was more taken aback by the substance of the exchange than by its tone. (Lynch did not respond to my repeated e-mail messages requesting comment.) “D.O.J. wanted this,” Diaz told me, “so we had to make up some reasons why we needed it.” Justice Department officials sent an affidavit to be signed by the Guantánamo commander asserting that some of the detainees had been trained to pass “coded messages in furtherance of terrorist operations” to comrades on the outside. Diaz and the intelligence officers were asked to show how 12 detainees from Kuwait (whose lawyers were challenging the visiting rules in court) might pull off such a plot. But the officers could find only three Kuwaitis who sounded plausibly dangerous enough, and even then, the administration’s claims were rejected by the court. “It was a reach,” Diaz recalled. “We were just throwing up these obstacles in the way of implementing the Rasul decision.”

For much of his adult life, Diaz was the person in his family most likely to do the right thing. He was the one who would come to the rescue when someone needed help, the one who got through college and graduate school, the one who often kept the peace. His parents divorced bitterly when Diaz was 6, and he spent the next years careering back and forth between them. As children, Diaz, his older sister and their two younger brothers slept for a time in a single bed, cooking their own meals and shopping for groceries when the food stamps arrived. “We couldn’t count on our parents,” his sister, Shari Bravo, said, “but we counted on each other.”

By seventh grade, Diaz had attended nine different schools. His one respite from the turmoil came a few years later, when he joined his father, who had moved out to California after getting a nursing degree at Purdue University. Remarried to a younger woman with children of her own, Robert Diaz had rented a comfortable ranch house outside Apple Valley, a middle-class community on the edge of Southern California’s Inland Empire. There was a swimming pool, open land and even a pair of horses. “It was kind of like paradise,” Diaz remembered.

But the idyll was brief. As Matthew finished ninth grade at Apple Valley High School, he answered the door to find sheriff’s deputies outside with a search warrant. An unusual number of elderly patients had been dying at two small hospitals where Robert Diaz had worked, many after suffering violent seizures. The deaths exposed a host of problems at one of the hospitals, which was subsequently closed by the state. But after exhuming some of the dead, prosecutors in Riverside County theorized that the patients were killed by large injections of the commonly used heart drug lidocaine. Robert, who was new to both hospitals, became the prime suspect.

Robert had never been in trouble with the law. No one had seen him inject the patients with lidocaine. Nor, despite the high levels of unmetabolised lidocaine in their bodies, was it certain they had been murdered. But Robert Diaz was the only nurse who was on duty when all of them died, and he sometimes carried preloaded syringes of lidocaine in his pocket. Two vials of the drug were found in the search of his home. (Robert said he had simply forgotten to empty his pockets before leaving work.) Prosecutors never offered a motive for the killings, but Diaz was arrested in November 1981 and charged with the murders of 12 patients.

“That’s when things started falling apart,” Matthew Diaz told me. At 16, he was left to fend for himself. He drifted back to Indiana, where his mother lived, but returned to California the next summer as his father’s trial approached. He soon dropped out of high school, found a job washing dishes and moved into a San Bernardino motel with a 28-year-old woman who had become his girlfriend.

Diaz stood by his father, but Robert Diaz’s legal defense was a debacle. Because he could not afford a private attorney, his case fell to a public defender’s office that was beset with dissension and budget problems. Robert’s attorneys persuaded him to forgo a jury trial and take his case before a judge — a move that was almost unheard of in a capital murder case.

With his trial looming, Robert Diaz suggested that his son consider enlisting in the military. Almost a year later, Matthew was serving in an air-defense artillery unit in West Germany when he saw a brief item about his father in the military newspaper Stars and Stripes. Robert Diaz had been convicted of all 12 murders. His public defender presented no new evidence or character witnesses in the penalty phase, noting simply that Diaz was only 46 years old and had saved the taxpayers money by not having a jury trial. On April 11, 1984, the judge sentenced Robert Diaz to die in the gas chamber.

Matthew took a leave to visit his father in prison, but after he returned to Germany, he began to distance himself from the troubles back home. Over the years, when friends and colleagues asked, Diaz would tell them that his father was out in San Francisco. If they didn’t ask, he didn’t volunteer. “I wouldn’t say we’d lost touch, but I just focused on my own stuff,” he said. “The things I was exposed to, it just kind of hardened me.”

A few years later, his father began sending him documents from his trial. Diaz collected the files in a big Tupperware bin. The more he read, he told me, the more convinced he became that his father was the innocent victim of an incompetent defense. As Robert Diaz languished on death row at San Quentin State Prison, his son became more interested in the law. While stationed at Fort Stewart, Ga., he took an associate’s degree in law-enforcement studies, then a bachelor’s degree in criminology. He decided to quit the Army become a police officer in Georgia.

As Diaz recounted his subsequent decision to go to law school instead, he remembered having loved the Jimmy Smits character, Victor Sifuentes, on the television show “L.A. Law” and being impressed by an Army lawyer who came to speak to one of his college classes. But Diaz’s wife at the time, Melissa Reed, said his choice was deeply influenced by his father’s fate. “He was just trying to do anything and everything he could think of” to help his father, she said, recalling that they considered moving to California so that Matthew could work with a lawyer who had taken up the appeal pro bono. Robert Diaz’s experience also shaped his son’s view of the law, Reed said. “He felt that there were still a lot of people out there who weren’t getting fair trials,” she said. “He’s always been for the underdog; he’s always been for helping those people that nobody else cared about.”

To his closest law-school friend, Diaz also confided his hope that he would one day be able to help win his father’s freedom. But in 1992, the California Supreme Court rejected Robert Diaz’s appeal by a vote of 6 to 1, finding “substantial evidence” for his original conviction and disputing that his trial defense was incompetent. His execution was stayed while a habeas corpus petition filed by his lawyers moved slowly through federal district court, where an evidentiary hearing is still pending. Matthew Diaz continued trying to help, carting the tub of his father’s legal documents with him from post to post.

As Diaz settled in at Guantánamo in the summer of 2004, the military leadership there was trying to show a more humane face to the world. Following the scandal at Abu Ghraib, journalists visiting the Cuba base were invited to watch from behind one-way mirrors as interrogators plied their prisoners with snacks and tea. “This is a wholly different environment,” the task-force commander, Brig. Gen. Jay W. Hood, told a reporter that summer.

In sifting the abuse complaints that prisoners and others had registered at Guantánamo, Diaz did not see mistreatment on the scale of Abu Ghraib. Some prisoners said they had been beaten by guards; some officials reported interrogations they considered abusive. But as the file of complaints grew, Diaz said, officials continued to maintain publicly that only a handful had been confirmed. “There was a lot of stuff in the past that should have been disclosed but was not,” he said.

Some of the legal uncertainty surrounding the Guantánamo prisoners was supposed to be resolved by the military commissions that began their proceedings that summer. But what Diaz saw of the initial hearings in late August did not reassure him. A former Army judge presiding over one tribunal seemed to Diaz to be bewildered about how to proceed. “Whatever issues the defense was raising, he didn’t have the answers,” Diaz recalled. “It was embarrassing.”

Among Diaz’s other responsibilities was to help manage the civilian lawyers who began flying down to visit prisoners. The first to arrive was Gitanjali Gutierrez, a young lawyer for two British detainees, Moazzam Begg and Feroz Abbasi. Gutierrez cut a swath at Guantánamo, but not in a way that endeared her to most of the government lawyers with whom she battled over access. A Pentagon official later described her to the F.B.I. as “pushy and deeply suspicious,” according to a copy of his statement obtained by The Times. Another lawyer in Diaz’s office called her “difficult to please and very stubborn.”

But Diaz liked her. He found her smart, and he was impressed by her commitment to her work. At least a couple of the other lawyers who saw them together at Guantánamo got the impression that Diaz had something of a crush on Gutierrez, a notion he dismissed as silly. But he asked her to keep him in mind if her clients were ever charged in the tribunals and she needed a uniformed lawyer to help in the defense. A month later, he e-mailed her to wish her a happy 34th birthday (his own 39th was the next day). She sounded a little surprised to hear from him but wrote back that she was returning to Guantánamo soon and would bring him a new book about the Abu Ghraib scandal, “Chain of Command.”

As the end of his tour approached, Diaz’s frustration was growing. The prisoner-abuse files that he and others had compiled now filled two large binders. One statement, from a senior F.B.I. official, suggested that the military authorities had ignored complaints from bureau agents about harsh interrogation techniques. Another recounted a detainee’s claim that a guard had thrown him to the ground and rubbed his face violently in the dirt after the prisoner spat at him. Diaz found the report credible — the file included a photograph of the prisoner’s mangled face — and was surprised that it was not included among the allegations that the military made public.

In the statements they would later make to F.B.I. investigators, Diaz’s colleagues at Guantánamo generally described him as professional, affable and laid-back. Some of them were more impressed by him than others. But few of his fellow officers had much sense of Diaz the iconoclast: the lawyer who disdained the continuing war in Iraq, who quietly avoided social gatherings with more gung-ho government lawyers and who sometimes broke away from the caste society of the military to hang out with Jamaican and Filipino laborers who worked on the base.

Diaz was careful not to challenge the way things were done, he told me, and discreet about his views on Guantánamo. “I pretty much kept my thoughts to myself,” he said. “I didn’t broadcast them.” To the wider military community, he could even sound a little gung-ho too. As his time at Guantánamo was winding down, his colleagues suggested to the public-affairs office that Diaz would make a good subject for a profile in the task-force newsletter, Behind the Wire. The resulting article, “Fifteen Minutes of Fame with Lt. Cmdr. Matt Diaz,” told of a Latino kid who “left life on the street at 17,” worked his way up the ranks and made good as a Navy lawyer.

“What do you like about Guantánamo?” the interviewer asked him.

“I like the mission,” he said. “For the most part, everybody is trying to do the right thing, and I like being part of that and contributing.”

One afternoon in July, as we sat at a picnic table in the sweltering visiting area of the Charleston brig, I asked if he really believed what he had said. He said he did, describing soldiers and officers who went out of their way to act decently toward the men who were being held as terrorists. “They were usually just too far down the chain to make any kind of difference,” Diaz said.

Diaz’s own inability to make a difference grated on him. Pentagon investigators who were preparing a report on Guantánamo abuses seemed to ignore some of the cases he helped assemble, he said. Despite the first visits to prisoners by civilian lawyers, little information about their treatment seemed to be getting out. On Nov. 8, 2004, a federal district judge shut down the military commissions, ruling that they violated international law. But the case then moved to a conservative appeals court, where a reversal was widely expected. “I felt like nothing was ever going to change,” Diaz told me.

On Dec. 21, the Pentagon copied him on a letter from Barbara Olshansky at the Center for Constitutional Rights. Nearly six months after the Supreme Court decision in Rasul, she was still asking the government for the names and nationalities of the detainees so that lawyers could file habeas petitions on their behalf. In a draft response, the administration wrote that the detainees had other ways to obtain representation.

While other military lawyers felt that the detention camp was finally starting to open up to outsiders, Diaz was appalled by what he saw as the government’s obstinacy. “No matter what the courts said, they would just keep stonewalling,” he said. “I knew that if I didn’t do anything, nobody else was going to.” Working late one night, he logged onto a secure internal database to see what lists he could find. It was easy; he could bring up the names 100 at a time. Diaz said later that he did not ponder how the information might be received in New York. “I thought they would either file a petition on behalf of those detainees or maybe contact their families,” he told me.

As he lay in bed at night, Diaz said, he thought about the risk he would be taking if he went ahead. Over the previous year, the military had prosecuted or disciplined several servicemen for taking classified materials off the island. Security had been tightened. The Guantánamo counterintelligence officer slept in the next bedroom of the town house Diaz shared with several midlevel officers. The career for which he had worked so hard would be on the line. He was within striking distance of a promotion to commander, or of retiring with an officer’s pension.

Diaz would later say he didn’t know the information he mailed off was classified. The lists he printed out were not marked “Secret,” although officials later acknowledged that they should have been. His lawyers emphasized that he had access to much more sensitive, top-secret information than anything he sent. Diaz also said he hadn’t known the meaning of all the alphanumeric codes that followed the names. But one of those codes identified the prisoners who had given information to Guantánamo interrogators. Military intelligence officials described that code — not the names — as the significant leak.

On May 18 this year, after a weeklong trial, a panel of seven naval officers convicted Diaz on four of five counts, including one of disclosing secret defense information that “could be used to the injury of the United States or to the advantage of a foreign nation.” By then, nearly two and a half years after Diaz had left Guantánamo, the politics of detention policy had shifted. The detainees’ names had been released under the Freedom of Information Act. The Supreme Court had ruled against the administration once more, upholding the minimum standards of the Geneva Conventions and derailing the military commissions. The president declared that he would like to close Guantánamo as soon as possible.

Diaz did not testify during the trial. But in a statement to the jurors before he was sentenced, he sounded overcome by remorse. “I didn’t want to make waves and jeopardize my career,” he told the jurors, who could have sent him to prison for 13 years. “I am disgraced. I am ashamed. I let the Navy down.” After three hours of further deliberation, the jurors issued a notably light sentence of six months’ imprisonment and dismissal from the military.

When we spoke a couple of months later at the brig in Charleston, Diaz was less contrite. He said he bore no resentment toward Olshansky and the Center for Constitutional Rights for turning his valentine over to the authorities; in fact, he was sending the group donations of $25 a month. Looking back, he insisted that he tried to do the right thing in the wrong way. “There was nothing else that I could really do,” he said. “I could have gone up the chain. But nothing I said would have ever left the island.”

Diaz is reviewing his own trial transcripts now — as he once reviewed his father’s — and working on an appeal with the same California lawyer who has handled his father’s appeals. Shortly before his scheduled release from the Charleston brig this month, he was stripped of his license to practice military law. He said he is unsure how he will support his family now but that he is thinking of trying to find work in legal aid, even if he is disbarred as a civilian lawyer too.

In November 2006, Diaz told me, he flew out to San Francisco alone and drove up to San Quentin. It was the first time he told his father about the charges he faced and the risk that he could be sent to prison himself. His father was pained by what had happened, Diaz said, but also proud that his son had tried to do what he thought was right.

“He understood,” Diaz said.

Correction: November 4, 2007

An article on Oct. 21 about Navy Lt. Cmdr. Matthew Diaz, who was court-martialed and convicted on some charges that involved making public the names of detainees at Guantánamo Bay, misspelled the name of a city where he lived as a teenager. It is San Bernardino, Calif., not San Bernadino.

www.nytimes.com/2007/10/21/magazine/21Diaz-t.html