If allowed to stand, the order would give terrorism suspects a powerful weapon to frustrate secret interrogations and jeopardize criminal prosecutions, prosecutors said in a pleading released Wednesday.
The Justice Department was expected Thursday to propose alternatives to the deposition that would attempt to break the legal deadlock over access to al Qaeda detainees.
In the filing, prosecutors are expected to suggest some form of limited access to information from the detainees, possibly including agreement that the detainees have made certain statements about Moussaoui's conduct.
The written brief was filed under seal last month before the 4th U.S. Circuit Court of Appeals in Richmond, Va. The January order by U.S. District Judge Leonie Brinkema, allowing the hookup, would give judges "a breathtaking right" to micromanage the fight against terrorism, prosecutors said.
Moussaoui, who is representing himself, has acknowledged that he is an al Qaeda loyalist but denies charges that he was a Sept. 11, 2001, conspirator as charged by the government.
Government sources have identified the captive as Ramzi Binalshibh, a suspected coordinator of the Sept. 11 attacks. Moussaoui argues that Binalshibh — whose name was removed from the public document — can testify that Moussaoui was not part of the plot.
Brinkema, in Alexandria, Va., said Moussaoui could question the witness with a brief time delay, allowing the court to prevent transmission of questions the court deems impermissible. Moussaoui is held in strict solitary confinement in the Alexandria Detention Center.
Brinkema several times has expressed her concern that the government may be denying Moussaoui information that could assist him as he represents himself. She recently questioned whether Moussaoui could receive a fair trial in open court, given the government's secrecy, and reminded prosecutors of their obligation to turn over material that could aid the defense.
An alternative to the open trial would be a military tribunal, which probably would allow greater secrecy and fewer rights for a defendant.
Direct questioning of an enemy combatant in the midst of the terrorism fight is beyond the court's powers and would interfere with an intelligence interrogation, the government said.
"Indeed, it would give terrorist defendants a powerful weapon to frustrate the executive's efforts in the struggle with al Qaeda," the government said, while ensuring the abandonment of prosecutions in the United States.
The government contended that Brinkema failed to consider alternatives to providing Moussaoui with direct access to Binalshibh, including a government statement summarizing the captive's now-classified responses to interrogators.
The appeals court has scheduled closed oral arguments for June 3, but signaled that it wanted a compromise on the access question. The court told Brinkema to invite the government to propose alternatives before the hearing, and she has done so.
A separate government brief opposed a motion by media representatives that the appeals court open the hearing and release some records now filed under secrecy.
The right of public access is substantially outweighed by the need to protect classified information, the prosecutors said, adding there was no practical way to divide oral argument between classified and unclassified portions.
Moussaoui also seeks to interview Khalid Shaikh Mohammed, a captive who is the suspected Sept. 11 mastermind. Only access to Binalshibh is believed to be before the appeals court.