Military prosecutors say they have tapes of telephone calls between the alleged mastermind of the Sept. 11 attacks, Khalid Shaikh Mohammed, and three of his accused co-conspirators talking in code about the plot months before it took place, a defense lawyer disclosed on Monday.

The lawyer, Jay Connell, revealed the existence of the tapes as part of a protest over plans to use them as evidence at the death penalty trial of the alleged conspirators. More than 17 and a half years after the attacks — in which 19 hijackers commandeered four commercial airplanes and crashed them into the World Trade Center, the Pentagon and a Pennsylvania field, killing nearly 3,000 people — there is still no date set for the trial in the proceedings at Guantánamo Bay.

Prosecutors gave defense lawyers the original audio and transcripts of their translation on Sept. 30, 2016, Mr. Connell said, and made clear they planned to use them at trial. Defense lawyers sought to investigate their origins and later discovered that the original trial judge, Army Col. James L. Pohl, had issued a secret order preventing them from knowing about the phone call collection system or asking questions about it.

Mr. Connell, who is representing Mr. Mohammed’s nephew, Ammar al-Baluchi, said that prosecutors secretly obtained a ruling in August 2018 from Colonel Pohl forbidding defense lawyers from learning how the phone calls were collected or investigating that question. The phone calls in at least two languages were made between April and October 2001.