U.S. prosecutors are refusing to produce a key witness in the case of Zacarias Moussaoui, the only man charged in connection with the September, 2001 terrorist attacks on New York and Washington. The stage may now be set to shift the Moussaoui case out of the civil court system to a military tribunal.

Attorney General John Ashcroft said he will not allow Zacarias Moussaoui to question Ramzi Binalshibh, considered by prosecutors a key al-Qaida operative who helped organize and fund the 2001 attacks.

Mr. Moussaoui is acting as his own attorney in his murder and conspiracy trial. He insisted that testimony from Mr. Binalshibh will clear him of involvement in the plot to attack the World Trade Center and the Pentagon.

But the Justice Department has notified the judge in the case that it will not allow access to Ramzi Binalshibh because it could damage national security. Mr. Binalshibh was captured in Pakistan last year and reportedly has been cooperating with investigators hunting other al-Qaida operatives.

The attorney general's refusal to provide access to the witness could cause the judge in the case to dismiss all charges against Zacarias Moussaoui. That in turn could cause the government to shift the case out of the civil court system to a military tribunal where defendants have fewer rights.

Mr. Moussaoui remains the only person charged with being part of the conspiracy that carried out the September 11, 2001 attacks. He was arrested a few weeks before the attacks in Minnesota where he had been attending a flight school.

Mr. Moussaoui has acknowledged he is a member of al-Qaida and is loyal to Osama bin Laden but has said he had nothing to do with the September 11 attacks.

His trial is taking place in Alexandria, Virginia, just outside Washington.