The Miami Herald first reported last week on the declassified statement, which prosecutors included in a court filing that has not yet been officially made public. The Pentagon publicly released the statement on Monday.
Mr. Nashiri was arraigned in 2011 over the Cole bombing, which killed 17 sailors. His case had dragged on through years of pretrial hearings until a military judge indefinitely shut it down last month because most of his defense lawyers had quit.
Although the details have been murky, the defense lawyers had made clear that they had found something that raised concerns about their ethical obligation to protect the confidentiality of their communications with their client. They also complained that they could not talk to Mr. Nashiri about it because the details were classified, and that the judge, Col. Vance Spath of the Air Force, rejected their request for an evidentiary hearing to investigate the problem.
Colonel Spath and prosecutors have portrayed the mass resignation by the defense lawyers as essentially a stunt strategically timed to delay or derail the case, suggesting there was no basis for their concerns. Because the basic details were classified until recently, the defense lawyers have been limited in what they could say to rebut those accusations.
But the claim that the government had never previously put forward the explanation for the microphone sheds new light on the intensity of the recent sparring. Lt. Alaric Piette of the Navy, the only remaining lawyer representing Mr. Nashiri, also said the newly declassified statement was the first time the government said that the microphone was a legacy device and that a wire leading from it into the wall was not hooked up to anything.
“If you look at the microphone, there is no indication it is unhooked. If it is legacy, you can’t tell,” he said, adding, “We had not heard anything of this before.”
The declassified statement and the interviews with the two defense lawyers also filled in other details about the highly disruptive incident in one of the most important war-crimes cases arising from the conflict with Al Qaeda.
For example, Mr. Kammen said Mr. Nashiri’s defense team had already grown suspicious that its conversations with its client might be subject to surreptitious monitoring because of an earlier incident, although he said he could not describe it because its details might be classified.
“There was at least one episode, I’m comfortable saying, that absolutely caused us to believe that there was ongoing eavesdropping,” he said.
In April 2017, the statement said, it came to light that confidential conversations involving other detainees and their lawyers elsewhere in the prison complex had been “unintentionally overheard.” As a result, Brig. Gen. John Baker, who oversees military commissions defense lawyers, sent an email in June warning all defense lawyers of confidentiality problems.
Days after that email, Mr. Kammen said, the government told defense lawyers that everything related to General Baker’s email was classified — a sweeping edict that raised their suspicions and prompted the search that found the microphone in August. After the discovery, the defense filed a classified motion asking Colonel Spath for an evidentiary hearing.
But the judge denied its request to investigate, finding that, as he put it in open court, “there wasn’t any basis to find there had been an intrusion into attorney-client communications between this accused and this defense team.” The judge also later dismissed their concerns as “fake news.”
Both lawyers said they were puzzled by Colonel Spath, who has indicated he may retire. Lieutenant Piette said that at one point, when the judge declared there was no evidence to support defense concerns about confidentiality, he grew worried that maybe the judge only knew about the earlier incident involving other detainees and other lawyers.
During a break in the hearing, he said, he ran back to his office to verify that the defense had included a statement of facts about the microphone discovery with its classified filings.