The government has declassified a secret intelligence program that clandestinely recorded prison yard conversations of Khalid Shaikh Mohammed, who is accused of being the mastermind of the Sept. 11, 2001, attacks, as prosecutors seek to supply evidence that could be used at an eventual trial.
The move comes as prosecutors have considered new ways to counter claims by defense lawyers that torture by the C.I.A. contaminated subsequent F.B.I. interrogations of Mr. Mohammed and his accused accomplices to produce confessions the government considers its most important trial evidence. It also shed light on an eavesdropping operation whose existence has until now never been formally acknowledged.
In court this week, an Iraqi American translator for the F.B.I. vouched for a transcript of Mr. Mohammed describing how he learned when the hijackers would strike. It came in a coded message, according to a fragment of the transcript shown in court: “Your friend so and so will be married, uh, on Sept. 11.”
The conversation was captured on a nearly hourlong recording of Mr. Mohammed speaking with another prisoner at Guantánamo Bay on Oct. 23, 2007. Under a classified prison program, more than a dozen suspected terrorists, who had been subjected to years of solitary confinement and tortured by the C.I.A., were granted an hour of recreation time in earshot of another isolated prisoner.
By then, Mr. Mohammed was approaching five years of detention without charge or a lawyer. He had been waterboarded, sexually humiliated, sleep deprived and questioned by American interrogators for years, including by F.B.I. agents in 2007.
Now, 16 years later, the prison yard recording has been made public.
The timing coincides with efforts by prosecutors in the Sept. 11 case to shore up their evidence after a damaging ruling in the other death penalty prosecution at Guantánamo, the Cole case. Both cases are in pretrial proceedings, with no date set for jury selection as the military judges decide what evidence can be used at a trial.
In August, a judge in the Cole case excluded confessions made by the defendant, Abd al-Rahim al-Nashiri, during F.B.I. interrogations at the wartime prison in 2007. The judge concluded the confessions were derived from torture, a decision that undermined a bedrock prosecution strategy in military commission cases at Guantánamo.
The government is appealing the ruling. But in response, prosecutors in the Sept. 11 case have told their judge, Col. Matthew N. McCall, that they would call more witnesses next year to reinforce their position that Mr. Mohammed and his co-defendants voluntarily self-incriminated in interviews with F.B.I. agents in 2007.
Then, this week, it was disclosed in court that the secret eavesdropping program had been declassified. Lawyers revealed that a government intelligence operation had secretly collected conversations of one prisoner shouting to another during recreation periods. It is not known when the operation was ended, if at all.
The program itself had been alluded to though not formally acknowledged. Across a decade of pretrial hearings, government witnesses would sometimes mention, mysteriously, the existence of transcripts of “conversations” but never disclosed how they were obtained.
James G. Connell, a lawyer for Mr. Mohammed’s nephew, Ammar al Baluchi, who is a defendant in the case, said he would challenge the admissibility of the audio recordings as derived from cruel, inhuman and degrading treatment.
From 2006 to 2009, he said, the prisoners were held in solitary confinement and each one was allowed to speak only with one other prisoner, during an hour reprieve from their maximum-security cells.
At the time, prisoners in the custody of a special unit known as Task Force Praetorian or Task Force Platinum were given recreation time in specifically designated pairs. They were confined to separate enclosures, meaning the prisoners could shout back and forth but not see one another.
Mr. Connell called the recordings the product of a government-devised system of solitary confinement that for years permitted prisoners “a single outlet to communicate with another human being.” The government had then “exploited it” for trial purposes, he added.
A senior F.B.I. official, Jacqueline Maguire, said in testimony last week that she thought the recreation yards were being bugged “for force protection,” essentially to listen in on detainees to safeguard prison staff.
In the instance of Mr. Mohammed, his recreation partner was Guled Hassan Duran. Mr. Duran, a Somali citizen, was captured in Djibouti in March 2004, and held by the C.I.A. before he was transferred to Guantánamo in September 2006. He has never been charged with a crime.
In the portion of their conversation that was shared in court this week, Mr. Mohammed explained that he learned that the attacks would take place on Sept. 11 from a visitor “who came to my house in Kandahar,” in Afghanistan.
From there, he said, he went to visit “the sheikh,” an apparent reference to Osama bin Laden, to relay the message. “There were 20 days left,” Mr. Mohammed said, according to the transcript. “We were on, uh, Aug. 20 or so.”