A federal appeals court ordered Justice Department lawyers yesterday to explain at a closed hearing why they provided "arguably inconsistent" information about the interrogation of al Qaeda detainees -- a subject that already has mired the case of terrorism suspect Zacarias Moussaoui.

The order by the U.S. Court of Appeals for the 4th Circuit comes just after a crucial decision by the same court on the same issue had moved the case forward. Yesterday's request, however, could once again slow the prosecution of the only person charged in the United States in connection with the Sept. 11, 2001, terror attacks.

The order came in response to a letter filed by the Justice Department that sought to clarify the role of prosecution team members in the secretive process of questioning the detainees. The letter says that some Moussaoui prosecutors and FBI agents are involved in broader terrorism investigations and have "shared information." But the public release of the letter is heavily redacted for national security reasons, and it could not be determined what type of information was shared or in what context.

The issue is potentially important because the 4th Circuit, in a recent ruling, said Moussaoui could not question the detainees and ordered that the jury at his trial be instructed that no one involved in the case had been "privy" to the process of gaining information from the captives. Moussaoui had sought to question them because he said they could clear him of involvement in the Sept. 11 attacks. His attorneys are appealing the ruling.

In its order released yesterday, a three-judge panel said the information in the government letter "is arguably inconsistent with statements previously made to the court." The order also was heavily redacted and it was unclear what the judges will ask prosecutors at the hearing, but they asked why the information in the letter was not shared with them earlier and demanded the "identity and role in the prosecution" of those involved in providing information.

Lawyers said it was hard to assess the significance of the development without knowing whether prosecutors were indeed involved in the interrogations. But if they were, the outside lawyers said the appellate court could view that as a violation of Moussaoui's constitutional rights because his attorneys weren't also allowed to question the detainees.

"It sounds like the judges are very troubled because the information they had been getting through the course of this litigation was not accurate," said Michael Greenberger, a Justice Department official in the Clinton administration who is now a law professor at the University of Maryland. "The entire premise of their ruling, that Moussaoui could get evidence he needs that is free of any prosecution taint, could turn out to be incorrect."

Attorney General John D. Ashcroft yesterday said the Justice Department welcomes the opportunity to address the issue at the closed hearing in Richmond, set for June 3. "We believe that the position of the department is consistent," he said at a news conference.

Other federal law enforcement sources downplayed the significance of the order and said the issue would not affect the case.

But defense attorneys for Moussaoui, in a letter filed with the 4th Circuit, said, "A fundamental factual premise upon which the defense has been operating up until now . . . is incorrect."

Moussaoui, a French citizen, was charged in December 2001 with conspiring with al Qaeda in the Sept. 11 attacks.

The issue of detainees became paramount when U.S. District Judge Leonie M. Brinkema in Alexandria ruled in January 2003 that defense attorneys could depose a captured al Qaeda operative, Ramzi Binalshibh, the self-described planner of the attacks. Moussaoui's attorneys said he had information vital for the defense; the government strongly objected to the deposition on national security grounds.

The issue escalated in September when Brinkema ordered depositions of two more detainees, including Khalid Sheik Mohammed, the former al Qaeda operations chief. When prosecutors refused to produce them for questioning, Brinkema punished them by striking the possibility of a death sentence for Moussaoui and eliminating any Sept. 11-related evidence from his trial.

The 4th Circuit panel last month overturned that ruling, restoring the death penalty and the Sept. 11 evidence and ruling that Moussaoui could not question the detainees.

The judges sent the case back to Brinkema to craft a middle ground in which Moussaoui could have access to certain statements made by the detainees and present them as evidence to the jury.