A federal judge has interrupted the sentencing trial of Zacarias Moussaoui, the only man to be charged in the US in connection with the 9/11 attacks.
The trial has been hearing from a number of witnesses
Judge Leonie Brinkema is considering whether to rule out the possibility of executing Moussaoui because of "egregious" government misconduct.
The government said one of its lawyers had coached four witnesses, breaking rules set by the judge.
Moussaoui has pleaded guilty to six charges of conspiracy.
The prosecution has called for the death penalty, but defence lawyers are seeking a term of life imprisonment.
Judge Brinkema said she had been advised by the prosecution that a lawyer for the Federal Aviation Administration (FAA) had breached her rule that no witness should hear trial testimony in advance.
The lawyer had read a transcript of the first day of the trial and discussed some of the testimony with four potential witnesses, Judge Brinkema said.
"In all the years I've been on the bench, I've never seen such an egregious violation of the court's rule on witnesses," she told the trial in Virginia, now into its second week.
Prosecutor David Novak said the FAA lawyer "should have known it was wrong".
The defence immediately filed a motion for the death penalty to be dismissed as a possible sentence. Defence lawyer Edward MacMahon said: "We are not going to get a fair trial".
The defence called for a mistrial last week after Judge Brinkema pulled up the prosecution over a line of questioning.
"This is the second significant error by the government affecting the constitutional rights of this defendant," Judge Brinkema said on Monday.
"More importantly, it affects the integrity of the criminal justice system of the United States."
The prosecution are seeking to prove that Moussaoui knew about the 9/11 plot and kept deliberately silent while he was being held in US detention.
The 37-year-old Frenchman of Moroccan origin was arrested shortly before the attacks on New York and Washington after arousing suspicion at a flying school.
He initially told federal agents he was training as a pilot only for personal enjoyment.
A self-confessed member of al-Qaeda, Moussaoui has said he was not meant to be part of the 9/11 attacks, but was part of a broader conspiracy to use airplanes to strike the White House.
The sentencing trial is expected to last up to three months.