The defense said Moussaoui must be allowed to question the witnesses in order to get a fair trial.
"What the defendant seeks is sort of a windfall from the … government's war on terrorism," deputy solicitor general Paul Clement told a federal appeals court.
The session before three judges from the 4th U.S. Circuit Court of Appeals was to be followed in the afternoon by a closed hearing in which classified matters concerning the al Qaeda prisoners were to be discussed.
Clement said that if Moussaoui were allowed to question the witnesses, they might invoke their Fifth Amendment right against self-incrimination and Moussaoui would be no better off than he is now.
Clements told the court that the commander in chief was being forced to choose between prosecuting alleged Sept. 11 conspirator Moussaoui and future terrorism cases, says CBS News Reporter Kathy Mountcastle.
Federal public defender Frank Dunham asked whether the government should be able to seek to execute a man for allegedly plotting the Sept. 11, 2001, attacks while barring him from witnesses who know all about those plans.
"The answer to that question is a resounding no," Dunham said. "Our system of American fair play and justice will not allow such a result."
In a case with far-reaching implications for the war on terror, the appeals court is hearing arguments on whether the government should be barred from seeking the death penalty for Moussaoui because it refused a judge's order to produce al Qaeda witnesses for his defense.
In several crucial pretrial rulings, U.S. District Judge Leonie Brinkema has sided with Moussaoui, who is charged with conspiring in the Sept. 11 attacks. Those rulings are the basis for the government's appeal.
Brinkema issued two separate orders that would allow the Moroccan-born French citizen access to three al Qaeda prisoners held abroad.
The government refused to make the captives available, citing national security.
Brinkema concluded the al Qaeda prisoners could help Moussaoui's defense by testifying he had no role or an insignificant part in planning the Sept. 11 attacks. Two of the captives are top al Qaeda operatives: Ramzi Binalshibh and Khalid Shaikh Mohammed.
To punish the prosecutors for refusing to make the captives available, Brinkema eliminated any evidence that could link Moussaoui to the suicide hijackings and barred the death penalty in his case.
No one in the courtroom Wednesday disputed the government's claim that allowing Moussaoui to question enemy combatants would jeopardize the war on terrorism. But judge Roger Gregory asked, "At what point is (the threat to) national security so great that a person can't get a fair trial?"
"That's the issue the judiciary system must decide," Gregory said.
The outcome of these pretrial disputes could determine the government's ability to prosecute terrorist suspects in civilian courts. If Moussaoui should win access to al Qaeda captives, future terrorism defendants could ask for the same consideration and probably force the government to drop some prosecutions or move them to military courts, where national security would be paramount.
The government says Moussaoui, 35, who admits to being a member of Osama bin Laden's network, is trying to injure national security indelibly by questioning al Qaeda captives, who may reveal classified information that could help other terrorists.
The defense contends the government is preventing Moussaoui from exercising his Sixth Amendment right to call favorable witnesses, who might back up his contention that he was not part of the Sept. 11 conspiracy.
Arrested a month before the attacks, Moussaoui contends he was to be part of a later al Qaeda operation. The government says he may have planned to fly a plane into the White House.
Moussaoui was charged in December 2001 with participating in a broad al Qaeda conspiracy to commit terrorism and hijack airplanes. The overt acts included, but were not limited to, planning the Sept. 11 attacks.
In a shift in its policy on prosecuting terrorism suspects, the Justice Department on Tuesday said it would allow one of the three men classified an "enemy combatant" — and held in a military prison without charge — to see a lawyer.
Moussaoui's dilemma is that if he wins his appeal, he could face the same designation as Hamdi. If he loses, he could face the death penalty.