GUANTANAMO BAY: Lawyers for the five Guantanamo Bay prisoners charged in the Sept. 11, 2001, attacks want to know if U.S. government officials have been eavesdropping on their private conversations with the defendants.    

The evidence for any such listening, the subject of a hearing scheduled to start Monday at this U.S. base in Cuba, is circumstantial.    At a hearing Jan. 28, the sound system in the Guantanamo courtroom was suddenly cut, to the surprise of even the judge. The judge later revealed that a government official, from an agency that the military has refused to disclose, was following the proceedings from outside the courtroom and intervened to prevent the potential release of classified information.

The judge, Army Col. James Pohl, later said the information was not classified and he ordered the undisclosed government agency to disconnect any equipment that could unilaterally cut the sound. He also released a transcript of the censored remarks.

But since the Jan. 28 incident, lawyers for the defendants say they have become more concerned about possible additional monitoring that they say would violate attorney-client privilege and make it impossible for them to represent men charged with aiding and planning the Sept. 11 attacks.

At the hearing, they will be asking the judge to halt all proceedings until the issue is resolved.

''What happened in the courtroom (on Jan. 28) was shocking,'' Army Capt. Jason Wright, one of the lawyers for lead defendant Khalid Sheikh Mohammed, said Sunday. ''There was a wizard behind the curtain who had the power to completely cut off the audio feed to the proceedings, to censor what was being said in court. It would be foolish for us to not consider that capability in other areas where we interact with the accused.''

One concern is the audio system inside the high-tech courtroom overlooking Guantanamo Bay. The microphones at each defense table are so sensitive that officials are apparently capable of hearing even whispered conversations between the defense lawyers and their clients, attorney James Connell said. Other lawyers said they are also worried about possible monitoring of their conversations in rooms where they meet with the defendants.

The chief prosecutor, Army Brig. Gen. Mark Martins, said there is no evidence of any monitoring.

''My staff and I spent a full week diligently running every rumor to ground, and I can say unequivocally that no entity of the United States government is listening to, monitoring or recording communications between the five accused and their counsel at any location,'' Martins said Sunday.

Khalid Sheikh Mohammed, the self-professed mastermind of the Sept. 11 attacks, and four other defendants are being tried by a military commission, which is a special tribunal for wartime offenses that includes elements of a court martial and a civilian court.

They face charges that include nearly 3,000 counts of murder and terrorism for their alleged roles planning and aiding the 2001 terrorist attacks and could get the death penalty if convicted.