A Prince William County police officer was shot and killed yesterday in the house of a Dale City man who was then slain by police in a violent Thanksgiving morning that apparently began in Arlington with the shooting of another officer.

Killed in Dale City was Philip "Mike" Pennington, 35, an 11-year member of the Prince William force, and Mark F. Arban, 31, who had lived alone in a single-family house there for about three years.

Arban, police say, was suspected of shooting and wounding Arlington Sheriff's Deputy Daryl F. LaClair during a roadside encounter early yesterday. A witness provided an automobile license number, which led officers to Arban's house, 15 miles away and about a mile southwest of the Potomac Mills shopping center.

Pennington was the first Prince William County officer to die from gunfire in the 20-year history of the department. The only other duty-related death resulted from a traffic accident in 1973.

"The department is stunned, just numb," said Prince William Police Chief Charlie T. Deane. Officers attached black bunting to their badges yesterday and flags were ordered flown at half-staff. Pennington and his wife, Connie, had a 6-year-old son, Philip.

"It's a real tragedy," said Paul B. Ebert, the commonwealth's attorney for Prince William. "Pennington was an outstanding officer. I think it's just an indication of the lack of respect a certain element has for police officers. I'm saddened beyond words."

Arban, who was unemployed, was described by neighbors in the Darbydale community of Dale City as a loner.

Kay Castro, 49, who lives two doors away, said, "I knew he was a little strange. He never came out during the day. He only came out at night. He ordered pizzas, morning, noon and night . . . . He never mowed the lawn, never did anything to the house. He was sort of like an eyesore."

Police have been at Arban's house several times in the last few months, neighbors said, and a police source said he had a record of minor crimes.

Mamie Roberts, another neighbor, said that Arban "just needed help and nobody helped him."

Precise details of the Dale City shooting remained sketchy last night. As pieced together from various law enforcement sources, including Chief Deane:

About four Special Weapons and Tactics (SWAT) officers, who had obtained a warrant in connection with the earlier Arlington incident, went to the front door and urged Arban to surrender. He refused to let them in and the officers broke down the door and entered the house.

After several minutes of discussion with the police, Arban, who was holding a pistol, apparently retreated and grabbed a high-powered, long-barreled weapon, perhaps an AK-47 assault rifle, fired, and mortally wounded Pennington.

SWAT team members returned the fire as officers retreated from the house to plan their next move. Because they were unsure whether Arban was alive, Deane said, officers shot tear gas into the house, then reentered the house after determining Arban was dead.

Police believe Arban was killed by the SWAT team's sniper, who was stationed outside the house.

Pennington was wearing SWAT equipment, including a helmet, bullet-proof vest and shield, but the bullet apparently pierced the helmet, authorities said.

The exchange of gunfire occurred about 6:30 a.m., police and neighbors said. Pennington was pronounced dead at 7:12 a.m. at Potomac Hospital in Woodbridge. Arban was declared dead at the house.

Neighbors said they believed officers were firing guns into the house over a 90-minute period, but they apparently were mistaken, Deane said. He suggested that the tear-gas firing could have been mistaken for weapon fire.

The Dale City shootings were preceded by events that began early yesterday in Arlington.

About midnight Wednesday, Deputy LaClair was driving his sheriff's vehicle west in the 6400 block of Lee Highway (Route 29) when a man whom police believe was Arban pulled alongside in a Buick Century and waved at the deputy to stop.

LaClair did so and flipped on emergency flashers. While he was walking to the Buick, police said, the man turned around in the seat, leaned out the driver's-side window, and fired a pistol at LaClair, striking him in the elbow and right eye. LaClair dropped to the ground and shot at the car, hitting the rear windshield.

The sheriff's deputy, a 10-year member of the department, was in fair condition at Arlington Hospital, where he said yesterday, "I'm just thankful to be okay. I thought the guy needed help, so I pulled over."

Witnesses jotted down the license number on the Buick, allowing police to trace it to Arban's Dale City address, where Arlington authorities found a Buick with a shot-out rear windshield in the driveway.

Prince William police were summoned, a warrant was obtained, and authorities surrounded Arban's house about 6 a.m.

As the police closed in on Arban, "They were saying they were not the KGB," said neighbor Marvin Roberts. "All they wanted to do was search his house."

Roberts's wife, Mamie, son Troy, 13, and daughter Megan, 4, crouched down on the bedroom floor while Roberts watched out the living room window.

Arban's house is a two-story brick rambler with black shutters; some windows appeared to be shot out yesterday. Real estate transaction records list Arban as the owner and say he bought the house in 1988. Neighbors describe the area as quiet and family-oriented.

A couple identified by police as Arban's parents came to the home yesterday afternoon.

Pennington was a graduate of Radford University in Radford, Va., and was described by colleagues as low-key, laconic and outgoing.

"He was outstanding," said Prince William Deputy Chief James K. Sullivan. "I really hurt. He's the first one I've lost."

Staff Writer Pierre Thomas contributed to this report.