Sept. 11 conspirator Zacarias Moussaoui's guilty plea was invalid because he was denied potentially helpful evidence and was restricted in choosing his own counsel, his lawyer told a federal appeals court Friday.

A federal prosecutor countered that Moussaoui got exactly what he wanted when he ignored his attorneys' advice and pleaded guilty before the evidence he had sought could be provided.

"He is the one who cut the process off at the knees by insisting on pleading guilty against the advice of counsel," said U.S. Justice Department attorney Kevin Gingras.

Moussaoui, the only person to stand trial in a U.S. court in the Sept. 11 attacks, is serving life in prison for helping plan the attacks. Gingras told a three-judge panel of the 4th U.S. Circuit Court of Appeals that Moussaoui "testified he rejoiced in the nearly 3,000 deaths that resulted from his conspiracy."

However, Moussaoui has since proclaimed his innocence and is seeking a new trial. The appeals court heard Moussaoui's case in January but ordered Friday's rehearing after one member of the panel, Judge Karen J. Williams, retired for health reasons before the case could be decided.

Judge Dennis Shedd replaced Williams on the panel, joining Judges William Traxler and Roger Gregory.

Moussaoui's lawyer, Justin Antonipillai, said his client's right to hire trial counsel of his choosing was violated because the judge required that any attorney involved in his defense undergo a national security background check. He said it's unconstitutional to require government approval of a defendant's hired or pro bono counsel.

He also argued that Moussaoui's rights were violated because his attorneys could not talk to him about potentially helpful evidence gleaned from classified material. The attorneys could urge Moussaoui not to plead guilty but could not tell him the basis for that advice.

"If the court interferes in communication between a defendant and his lawyer, that's a dangerous thing," Antonipillai said.

Gingras said Moussaoui had a general idea about they type of evidence his lawyers had obtained because he had asked for it. But Moussaoui decided to plead guilty before the evidence could be provided.

"The idea that he was completely in the dark is simply not the case," Gingras said.

He also said the government's position is affirmed by Moussaoui's own words, spoken after the trial judge made it clear that a guilty plea would mean life in prison. Moussaoui said: "That was my choice."

After the hourlong hearing in open court, the judges and lawyers met in closed session to discuss classified matters.

The court usually takes several weeks, or even months, to issue a ruling.

Click here for more on this report from MYFOXNY.com.