WASHINGTON -- A new book says Justice Department prosecutors were stunned to learn three years ago that the U.S. military had secretly recorded incriminating comments by alleged Sept. 11 mastermind Khalid Sheik Mohammed 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 Sept. 11, 2001 attacks against the United States.

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 Guantánamo 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.