An Al Qaeda prisoner's reported description of Zacarias Moussaoui as merely a backup figure could weaken the government's argument for executing the man accused as a Sept. 11 conspirator, legal experts said Wednesday.

U.S. officials said Tuesday that a top Al Qaeda operative, Ramzi Binalshibh, told interrogators that Moussaoui met with the suspected mastermind of the Sept. 11 attacks in late 2000 or early 2001 in Afghanistan.

Binalshibh, a former aide to top Al Qaeda operative Khalid Shaikh Mohammed, said Mohammed provided Moussaoui with contacts in the United States — but the two men were not confident that Moussaoui could keep a secret, said officials speaking on the condition of anonymity.

Moussaoui, acting as his own lawyer, and a defense team appointed by the judge have asked that Binalshibh be a defense witness. The request is complicated in Moussaoui's case because the government may not want enemy combatants like Binalshibh to reveal sensitive information in courtrooms.

Legal experts said Binalshibh's reported statements posed a threat to the prosecution's stated goal of executing Moussaoui if he's convicted. The experts were less certain about the effect on the guilt or innocence phase without more details about possible Moussaoui knowledge of the Sept. 11 plot. Moussaoui has admitted his loyalty to Al Qaeda but has denied a role in the hijackings.

David Bruck, a Columbia, S.C., lawyer who heads a death penalty project for federal public defenders, said the federal death penalty law requires the government to prove a defendant's intention to kill — and that victims died as a result of his actions.

Moussaoui, who was arrested at a Minneapolis area flight school in August 2001, was in custody on Sept. 11.

"The statute was intended to cover a person who hires a hit man," Bruck said. "Being a stumblebum on the fringes, who was too unreliable to be trusted and then abandoned by plotters, is not committing an act that leads to anybody's death," Bruck said.

Stephen Bright, director of the Southern Center for Human Rights in Atlanta, said the Supreme Court has made it clear that major participation in taking a life is necessary for imposition of the death penalty.

In a 1982 Florida case, the court ruled that the role of a getaway car's driver was tangential to the deaths of a couple murdered during a robbery by accomplices.

Five years later, in Tison v. Arizona, the court upheld the death penalty for two brothers who brought weapons into an Arizona prison to help their father escape.

Several days later, the brothers and the father kidnapped a family and drove the victims into the desert, where the father killed the hostages after the sons had walked away.

The court said the Tison brothers' degree of participation was major, unlike the driver in the Florida case.

Robert Precht, an assistant dean at the University of Michigan's law school, and the lawyer for a defendant in the first World Trade Center bombing case, said it's difficult to determine whether Binalshibh's evidence could be exculpatory — tending to clear Moussaoui of guilt.

"If he was kept in the dark and was a backup person and the plan was to tell him to kind of cool his heels for next six months, and call him if they need him, that would be exculpatory because they're keeping him in the dark about the conspiracy," Precht said.

"If they briefed Moussaoui on the general aims, made sure he's trained and ready to go on a moment's notice, that's not exculpatory evidence."

Binalshibh was an associate of Mohammed, Moussaoui, chief hijacker Mohamed Atta and others in the Hamburg, Germany-based cell that led the attacks. He was captured by U.S. and Pakistani authorities during a raid in Karachi on Sept. 11, 2002, and has since been held in U.S. custody at a secret location.

The Moussaoui-Mohammed contacts were first reported Wednesday by The Washington Post.

Moussaoui's trial is scheduled for June.