A defense attorney for John Allen Muhammad said yesterday that he may try to call Fairfax County's chief prosecutor to the stand when the convicted sniper goes on trial in the slaying of FBI analyst Linda Franklin outside a Home Depot store.

After all, lawyer Jonathan Shapiro said, the prosecutor may be the best witness for the defense. He has already told one jury that Muhammad did not kill Franklin.

When prosecuting Lee Boyd Malvo last year in Franklin's slaying, Fairfax Commonwealth's Attorney Robert F. Horan Jr. called Muhammad all kinds of things -- vicious, brutal, uncaring -- but not the person who pulled the trigger.

"Talk about John Muhammad all you want," Horan told a Chesapeake jury last fall. "Maybe it was his plan. Maybe it was his idea, but the evidence stamps this defendant [Malvo] as the shooter. The evidence stamps this defendant as the killer."

Horan's closing argument during the penalty phase of Malvo's trial not only will become part of the legal strategy for Muhammad's attorneys, but it is also being cited by those questioning whether the older sniper should even face a second costly trial in Virginia.

"If several people are charged with a crime, the only one who can get death is the one who is the 'principal in the first degree' -- the one who commits the killing," said Ira P. Robbins, an American University law school professor. "If the government can't prove who pulled the trigger, then you may not be able to pin down who was the principal in the first degree. It's one of the big issues."

Prosecutors at Muhammad's first trial overcame that hurdle when Prince William Circuit Court Judge LeRoy F. Millette Jr. ruled that Muhammad could be eligible for the death penalty in the slaying of Dean H. Meyers, 53, even if he did not fire the fatal shot. That ruling, along with a conviction under a new and untested anti-terrorism law, will form the backbone of Muhammad's appeals.

Horan said this week that he intends to prosecute Muhammad, 43, in Franklin's slaying. The chief prosecutor has said that he wants to secure another death sentence as "an insurance policy" in case Muhammad's conviction in Meyers's slaying was overturned on appeal.

But legal experts said yesterday that Horan would have to rely on the same theories used by prosecutors in Muhammad's first trial. If an appeals court overturned the first conviction, it would set a precedent for the second, should Horan secure one. Law professors pointed to the volatility of Virginia's so-called triggerman rule, which typically has required enough evidence to prove that someone pulled the trigger.

Robbins also said the state's anti-terrorism statute, which makes it a capital offense to commit murder to influence or intimidate the public and government, could face trouble with the state Supreme Court. Prosecutors have said that the terrorism statute fits because Muhammad and Malvo allegedly tried to extort $10 million from the government to stop the killing.

"It's not anything like [the terrorist attacks] of 9/11," Robbins said. "I think there's a good argument, not necessarily a winning one, that it was not intended to address these kind of heinous acts . . . along the lines committed by Malvo and Muhammad. Judges have to go by the wording of the law. If it's ambiguous, they can look to the legislature."

Stephen Saltzburg, a law professor at George Washington University, agreed that a second prosecution is premature in Virginia: "Aren't they going to have the same problems in this case? If they go forward with these theories in a new case, there may be other errors."

Muhammad's other defense attorney, Peter D. Greenspun, said the defense team is requesting to file a 350-page appeal to the state Supreme Court -- 300 pages more than allowed -- listing what it says are more than a hundred errors in his first trial.

Other legal observers said they could not understand why Horan would pursue another costly death penalty case, particularly when some family members of victims have said they are not in favor of it. The price tag for the first two sniper trials was nearly $3 million.

"You can only kill a person once," said Len Piotrowski, a lawyer with the Northern Virginia capital defender office, based in Manassas. "Why go through the tremendous expense of a second trial? Is Horan saying prosecutors didn't do it right the first time?"

Horan did not return calls yesterday. But on Tuesday, he cited the gravity of the October 2002 sniper attacks, in which 13 people in the Washington region were shot -- 10 fatally -- during a three-week reign of terror.

Bob Bushnell, president of the Virginia Association of Commonwealth's Attorneys, agreed that the seriousness of the acts justified the high cost for another trial.

"I would not second-guess Bob Horan on his decision to pursue [the perpetrators] of the most heinous and serious crime in the history of Virginia," Bushnell said.

Malvo and Muhammad are also charged in Alabama, Louisiana, Maryland and the District. Douglas F. Gansler, chief prosecutor in Montgomery County, where six people were killed in the sniper attacks, said he does not object to Horan's move, but said he, too, would like to prosecute the snipers. "You could have six families there . . . having their day in court," he said.

A spokeswoman for Virginia Gov. Mark R. Warner (D) said the governor would also not second-guess Horan's decision.

"Commonwealth's Attorney Horan sees this as the best way to ensure justice is done in this case and is in the best interests of the victims and their families," said Ellen Qualls, press secretary for Warner.

Staff writer Michael D. Shear contributed to this report.

The defense plans to use prosecutor Horan's earlier words to its advantage.