The self-styled mastermind of the September 11 attacks and four co-defendants have told a military judge at Guantanamo they want to confess and plead guilty.
The judge said he would question the five, including Khalid Sheikh Mohammed, who has already said he planned the September 11 attacks, to ensure they understood the impact of their decision.
All five could face the death penalty.
The judge, Army Colonel Stephen Henley, said he would not accept any guilty pleas during the hearings scheduled this week but did not explain why.
He read from the defendants' note, which began: "We all five have reached an agreement to request from the commission an immediate hearing session in order to announce our confessions ... with our earnest desire in this regard without being under any kind of pressure, threat, intimidations or promise from any party."
The note said all five wished to plead guilty and withdraw all pending motions filed by their military-appointed lawyers, whom they do not trust and have tried to fire.
The note said the defendants made the decision on 4 November, which was the day Barack Obama was elected to become the next US president.
Mr Obama has said he will shut down the widely condemned Guantanamo prison camp and try detainees in the regular US civilian or military courts.
Several of the defendants had said at previous hearings that they welcomed martyrdom, and they may have felt that opportunity slipping away with Mr Obama's election.
The announcement came as a surprise as the US military resumed pretrial hearings at the Guantanamo naval base.
The judge questioned whether the law underpinning the Guantanamo tribunals allowed him to accept a guilty plea in a capital case.
If the defendants are allowed to plead guilty, the case would still go through several automatic appeals, so any death sentence may not be carried out for years.
