Attorneys for convicted sniper John Allen Muhammad launched the first arguments against Virginia's anti-terrorism law before the state Supreme Court on Tuesday, saying it could not be used to sentence Muhammad to death because he was not the one who pulled the trigger.

Several of the seven justices seemed skeptical of that legal argument.

The defense also argued that the factual evidence at trial last year was insufficient to convict Muhammad, 43, of two counts of capital murder in the Prince William County slaying of Dean H. Meyers, 53, one of 10 Washington area sniper killings in October 2002. Defense attorneys Peter D. Greenspun and Jonathan Shapiro also renewed their claim that the prosecution of co-conspirator Lee Boyd Malvo, now 19, improperly used a conflicting theory of the same crimes.

The defense and the state attorney general's office were given 45 minutes each to argue their side of Muhammad's case, which was appealed automatically to the Supreme Court after Muhammad received a death sentence in March. A ruling is expected in January.

After the Sept. 11, 2001, terrorist attacks, the Virginia legislature passed an anti-terrorism law making it a capital offense to commit murder with the intent to intimidate the civilian population or influence the conduct of government. Prince William prosecutors used the law for the first time during the six-week trial in Virginia Beach last fall, arguing that the fear inflicted by the random attacks and the snipers' demand for a $10 million payment to stop the shootings fulfilled both prongs of the law.

Prosecutors acknowledged that they had no direct evidence that Muhammad was the one who fired the Bushmaster rifle tied to the killings, even in Meyers's case. Instead, they argued that Muhammad's role as an equal member of the sniper team, and as one who directed and controlled Malvo, qualified him for death even if he was not the triggerman.

Muhammad's attorneys argued that that theory was contrary to established Virginia law, which held that only the actual gunman could be convicted of capital murder. Senior Assistant Attorneys General Katherine P. Baldwin and Robert Q. Harris strongly disagreed, citing a 1991 Supreme Court ruling that "when two or more persons took a direct part in inflicting fatal injuries, each participant in the murder was an immediate perpetrator for purposes of the capital murder statutes."

Justice Donald W. Lemons cited that case to Greenspun, saying that one defendant held the victim down while another inflicted a fatal blow with a rock. "We have embraced a 'unit theory' of operation in cases in the past," Lemons said, meaning that two people acting as a single unit can both be held responsible.

"That's when you have two people engaged in the physical act," Greenspun responded. "Here the commonwealth has no evidence of two people holding the gun."

Chief Justice Leroy R. Hassell asked Greenspun, "Your position is the defendant has to be a participant in the physical act that results in the killing?"

Greenspun answered, "That's correct."

Baldwin reminded the justices that Muhammad and Malvo acted as a team, citing expert testimony from a British military trainer that effective snipers need a spotter to monitor the surrounding area, protect the gunman and signal when the time is right for a shot.

"This was all very joint participatory, hands on," Baldwin said. "In this case, it took two people. It took one to line up and call the shot, direct it, the other to shoot." She acknowledged that it was not clear who fired the shot that killed Meyers but said that it did not have to be.

Lemons asked whether the case qualified as first-degree murder without the sniper team or "unit" theory. "Absent the unit theory," Lemons asked Baldwin, "what evidence do you have? Does this case survive without the unit theory?"

Baldwin said it did. "They can have acted together . . . for this particular shooting. One person to line up the car, one person to line up the shooting."

Greenspun noted that the evidence linking Muhammad to Meyers's killing was thin. Muhammad was spotted in the area before and after the shooting at a Sunoco gas station on Route 234 near Manassas, but no one saw the shot fired, and Muhammad's fingerprints were not on the murder weapon.

Several justices asked Baldwin about the physical evidence: how many shootings took place outside the Chevrolet Caprice, the snipers' alleged lair, how long the gunpowder residue was in the car's trunk, and what evidence showed that the bullet that killed Meyers came from the Caprice. Baldwin acknowledged, "There is no direct evidence. This is a circumstantial case."

Shapiro argued that while prosecutors in Muhammad's trial said he was the mastermind of the sniper attacks, prosecutors in Malvo's trial said Malvo was "a free agent, a free thinker, nobody's fool." Shapiro said that the two prosecution theories "could not have been more contradictory" and that that violated due process saying that prosecutors may not use different theories of the same event.

Justice Cynthia D. Kinser noted that Fairfax prosecutors, in their argument that Malvo was a free thinker, were responding to a defense claim that Malvo was temporarily insane. "What's inconsistent," Kinser asked, "in defending against an insanity defense, by showing that Malvo still had the ability to make a decision on his own, and in this case showing that Muhammad was the dominant figure?"

Shapiro said the prosecution's own expert testified that Malvo told him he was not impressionable or weak. Lemons said people on sports teams or in offices take direction from coaches or bosses. "How's that inconsistent with a person retaining his ability to act and his willingness to take direction?" the justice asked.

Prince William Commonwealth's Attorney Paul B. Ebert watched the argument from the front row, along with assistant prosecutors Richard A. Conway and James A. Willett. Relatives of the victim did not attend. Although Ebert has prosecuted more than a dozen death penalty cases, he said it was the first time he had watched one argued at the Supreme Court.

He was hesitant to draw conclusions about the court's direction after the arguments. "Sometimes you think they're with you, sometimes against you," Ebert said. "You never know what they're thinking."