By the time the prisoner accused of plotting the U.S.S. Cole bombing boasted about his role in the attack during interrogations at Guantánamo Bay, his memories and account were unreliable because of years of isolation and torture by the C.I.A., a former military interrogator testified Friday.

Prosecutors say the statements that Abd al-Rahim al-Nashiri, a Saudi prisoner, gave during interrogations in 2007 are crucial evidence against him. Defense lawyers consider them tainted by torture. Now the judge, Col. Lanny J. Acosta Jr., is expected to decide whether agents can testify about the confession at Mr. Nashiri’s eventual trial.

The judge’s ruling is on track to be the first major decision at the war court about the admissibility of interrogations by federal agents who were brought to Guantánamo Bay to build a fresh case against former C.I.A. prisoners.

The final expert on the topic testified Friday that, no matter how friendly the so-called clean team of F.B.I. and Navy intelligence agents were, the legacy of Mr. Nashiri’s torture and years of C.I.A. detention made what the prisoner told them untrustworthy.

“The debility, dependency and dread doesn’t disappear when they walk into a clean room in suits,” said Steven M. Kleinman, who served in the C.I.A. and then the Air Force from 1983 to 2015 and retired as a colonel with a specialty in human intelligence.

Mr. Kleinman said prolonged isolation, sleep deprivation and brutality like that experienced by the C.I.A. prisoners degrades memory and leads to false confessions. Such treatment impairs a prisoner’s “ability to answer reliably” even years later, he said, adding that a prisoner “may be willing but is no longer able to correctly recall events.”

To a question from the judge, he said that U.S. law enforcement experience has shown that isolation and sleep deprivation have coerced prisoners to confess, and DNA evidence has discredited the confessions.

Mr. Kleinman capped months of expert and eyewitness testimony on whether Mr. Nashiri freely described his role in the suicide attack by Al Qaeda off Yemen that killed 17 U.S. sailors on Oct. 12, 2000. In April, a forensic psychiatrist testified for the government that, based on his reading of prison records and other information, Mr. Nashiri had voluntarily confessed.

Neither expert ever met or observed the prisoner.

Military doctors have diagnosed Mr. Nashiri with post-traumatic stress disorder and depression.

To get him to talk about Al Qaeda after his capture in 2002, C.I.A. employees in overseas prisons waterboarded him, confined him nude inside a chilled, cramped box, slammed his head in a wall. They also used solitary confinement and rectal abuse to keep him cooperative.

Then in 2006, the C.I.A. moved him to Guantánamo Bay on orders from President George W. Bush to put him on trial. Four months later, the “clean team” of federal agents questioned him in what they earlier testified were nonthreatening, friendly encounters.

One agent testified that Mr. Nashiri appeared unafraid and was proud of his work for Osama bin Laden on the Cole bombing. No recordings were made, but the agents wrote up an account as trial evidence.

The judge has said he wants to resolve the challenge to the confession before he retires from the military on Sept. 30 and scheduled final arguments on that question for later this month. As an added complication, Colonel Acosta is currently forbidden from issuing that and other key pretrial rulings.

The F.B.I. and Navy agents and others who observed Mr. Nashiri’s 2007 interrogation said that the atmosphere was friendly and that the prisoner self-incriminated. Mr. Kleinman said that from the prisoner’s perspective, the friendly agents of a government that had tortured him likely appeared “pretty callous” by not asking about his previous torture.

Defense lawyers chose Mr. Kleinman because he worked in an Air Force program known as SERE, for Survival, Evasion, Resistance and Escape. It teaches American pilots, commandos and other forces at risk of capture by the enemy how to survive torture and other brutality by using torture techniques that American prisoners of war were subjected to by Chinese, North Korean and North Vietnamese troops.

Mock interrogations at the SERE program were “very intense,” Mr. Kleinman said, but the U.S. service members knew their fake interrogators were Americans who would stop short of killing them on the waterboard. They were given a safe word to stop the interrogations, and there were multiple levels of supervision to avoid “abusive drift.”

Moreover, he said, the goal was not to gather intelligence but to strengthen a service member’s resilience.

“When you’re a detainee, you don’t know when it’s over,” he said. “You don’t know how far they’re going to go.”

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