Mark F. Arban used a military-type assault weapon he bought legally in a local store two years ago to kill Prince William County police Officer Philip Pennington on Thanksgiving Day, law enforcement sources said yesterday.

Pennington, 35, an 11-year member of the department, died after he was shot once in the head with an AKS-762 semiautomatic assault rifle, a Chinese-made replica of the Russian AK-47, a source said.

Since their initial statements after the shootings, Prince William police officials have refused to answer questions about the weapon used by Arban and other details of the incident. They plan to hold a news conference today.

A source said records indicate that Arban bought the rifle and a pistol, also found at his home, where the shooting took place, from a licensed gun dealer in the area about the same time in late 1988 or early 1989.

A gun dealer in nearby Woodbridge that sold semiautomatic weapons like the one that killed Pennington was closed temporarily in 1989 when federal officials arrested the owner for illegal gun sales. It could not be confirmed whether Arban bought his gun there.

The owner of Ted's Coins-Guns-Pawnbroker, former U.S. Park Police officer Donald P. Percival, then 40, and an employee, former Secret Service officer James S. Chapman, 44, were sentenced to jail for illegally selling dozens of semiautomatics to people who passed them on to Dwayne K. Boger, 18, of the District. Boger resold the guns or traded them for drugs, authorities said.

According to a search warrant, police also found in Arban's Dale City home a .25-caliber semiautomatic pistol, believed to be the weapon he used to shoot Capt. Daryl F. LaClair of the Arlington Sheriff's Department about six hours before the Pennington slaying. A 9mm semiautomatic carbine, four pellet guns and several rounds of ammunition also were found in the house.

Police and prosecutors have said that Arban had a history of mental illness, and the search warrant shows that a prescription bottle marked Desipramine, a commonly prescribed anti-depressant, was found in the house in the 14700 block of Dodson Drive.

The records also show that on the previous evening, Arban was seen outside an Alexandria restaurant, writing down license plate numbers of police cars parked there.

Alexandria Police Chief Charles E. Samarra said that officers saw Arban at 9:36 p.m. and ran a check of the license plate on his 1986 gold Buick sedan, learning that the car was not stolen and that he was not wanted by police.

After the announcement of Pennington's shooting, Samarra said the Alexandria officers were startled to discover that they, too, had encountered Arban.

"Certainly when he shot the officer in Arlington it was no accident -- he did that on purpose," Samarra said. "I have no idea what works in his mind, but he wanted to hurt some officer."

LaClair, the Arlington officer who was shot once in the eye and in the elbow, was patrolling in his car on Lee Highway about midnight Nov. 21 when a car believed to be Arban's pulled alongside him and the driver motioned him to pull over. When LaClair left the car, a man later identified as Arban pulled a gun and shot him. LaClair has been released from the hospital.

Arban, 31, was fatally shot by police about 6:30 a.m. on Thanksgiving Day, shortly after he shot Pennington, who was at the house with other Prince William officers trying to execute a search warrant in connection with LaClair's shooting.