WASHINGTON (AP) — A new book says Justice Department prosecutors were stunned to learn three years ago that the U.S. military had secretly tape recorded incriminating comments that alleged 9/11 mastermind Khalid Sheikh Mohammed made to fellow detainees during daily prison yard conversations but was not planning to use them at military tribunals.

In “Kill or Capture: The War on Terror and the Soul of the Obama Presidency,” journalist Daniel Klaidman says Mohammed was caught on tape boasting to other detainees about the 9/11 attacks. According to the book, Mohammed mentioned specific pieces of evidence, documents and computer files that could be tied directly to him through his voluntary statements to other detainees at the military detention facility in Guantanamo Bay.

Justice prosecutors were surprised because civilian prosecutors regularly use the jailhouse statements of inmates against them at trial and because the statements, voluntarily uttered, would allow the government to get around the problem of using statements the detainees made during harsh interrogations that defense lawyers would try to exclude from trial as tainted by torture.

Mohammed’s conversations “were intercepted by military spies and mined for intelligence,” Klaidman writes in his new book. “There were hundreds of hours of such recordings, including musings by KSM and other high-value detainees, uttered freely, during unguarded moments.”

It is unclear whether the military has changed its mind and now plans to use the recordings against Mohammed at his upcoming military commission trial. On Wednesday, a Pentagon spokesman, Army Lt. Col. Todd Breasseale, declined to comment.

Klaidman writes that despite “the potential gold mine” the recordings represented, military prosecutors decided a number of years ago not to use the evidence.

In fact, “they refused to even listen to the recordings,” Klaidman writes. “They worried that the intrusive means by which the evidence was obtained might not pass muster with their judges.”

Military tribunals were barely four years old at the time, largely untested and with practically no case law built up to guide lawyers, the military prosecutors were reluctant to take any chances, Klaidman writes.

In 2009 when the Justice Department was contemplating prosecuting KSM in a civilian federal court in New York City, prosecutors were eager to use the recordings, according to Klaidman’s book.

David Raskin, chief prosecutor of the terrorism unit in the southern district of New York, realized that “if KSM had talked openly about his role in 9/11, those statements would be among the most powerful evidence prosecutors could bring before a jury,” writes Klaidman. “They would be entirely voluntary statements, making them almost certainly admissible in court. Significantly, Raskin realized, prosecutors would be able to avoid a legal fight … KSM’s lawyers would not be able to, in effect, put the CIA on trial for torture.”

“For Raskin, the military lawyers’ refusal to listen to the tapes bordered on legal malpractice,” Klaidman writes.

According to the book, when Attorney General Eric Holder received a report on the military’s decision not to use the intercepted conversations, his first reaction was skepticism.

How could it be that the military was refusing to use this evidence?

“What am I missing here?” he asked.

Without explicitly disclosing the existence of the tape recordings, the attorney general made a cryptic reference to the evidence on them during a news conference 2½ years ago as he announced plans to try Mohammed and other terrorist suspects in a civilian court, the book says. The Obama later administration dropped the planned federal court trial in the face of widespread political and public opposition.

On Nov. 13, 2009 when the attorney general announced the planned trial, “Holder stubbornly stayed on message, saying over and over again that their case was strong and he was confident the Justice Department would win convictions,” writes Klaidman. “Then he obliquely alluded to the secret evidence that he believed was his trump card: ‘I will say that I have access to information that has not been publicly released that gives me great confidence that we will be successful in the prosecution of these cases in federal court.'”

From a legal perspective, the recordings were a vast improvement over evidence obtained through harsh interrogation techniques inflicted on Mohammed by CIA interrogators.

Mohammed was water boarded 183 times during his interrogations. In contrast, his conversations with other inmates would be free of the legal taint resulting from a variety of harsh interrogation techniques, which included pouring water over the immobilized prisoner’s face to impose the sensation of drowning.

The alternative was for military prosecutors to use clean-team statements, interviews conducted by FBI interrogators with no involvement in the CIA’s enhanced interrogation program, but some federal judges had thrown out clean-team evidence as well, arguing that once a defendant had been tortured, any future testimony he might give to American authorities would be tainted, Klaidman writes.

James Connell, an attorney for Ali Abd al-Aziz Ali, one of the defendants in the Sept. 11 trial at Guantanamo, said that he was unaware of any recordings, but that if it turns out that they exist he would ask for them to be turned over to the defense to be evaluated. Any statements made by a defendant that are in the possession of the government would be subject to discovery under military commission rules, Connell said.

Cheryl Bormann, an attorney for another of the defendants, Walid bin Attash, said she wouldn’t be surprised if the government had surreptitiously recorded her client’s conversations.

“Their entire scheme here, meaning the U.S. government, has been to collect intelligence,” Bormann said. “The system was designed to do that and prosecution was an afterthought.”

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Associated Press writer Ben Fox in San Juan, Puerto Rico, contributed to this report.