The self-styled mastermind of the September 11th attacks and four co-defendants said in note to a military judge at Guantanamo they wanted 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 11th attacks "from A to Z," 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.
"I am not trusting any Americans," Mohammed said in English during an appearance before the judge.
He and several co-defendants had said in an earlier hearing that they welcomed martyrdom.
But the announcement came as a surprise as the US military resumed pretrial hearings at the Guantanamo naval base, in a remote US-controlled corner of Cuba, for the accused plotters of the September 11th attacks.
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 the any death sentence would likely not be carried out for years.
The hearings went forward as scheduled, even though the pending change in the US administration made it unlikely the defendants' trials would ever be held at the base.
US President-elect Barack Obama has said he will shut down the widely condemned Guantanamo prison camp and try detainees in the regular US civilian or military courts rather than the special Guantanamo tribunals created by the Bush administration.