A veteran Prince William County police dog became the department's first canine killed in the line of duty after an officer accidentally shot him while trying to capture a suspect, authorities said yesterday.

Police believe Gunner, a 7-year-old German shepherd, was doing his job as he guarded the front door of a Woodbridge townhouse that his handler had just entered late Monday in pursuit of a drunken driving suspect. The dog blocked the door when other officers arrived on the scene. Not recognizing Gunner as a police dog, one of the officers fatally shot him, said 1st Sgt. Kim Chinn, a police spokeswoman.

The officer has not been identified, and the incident is under investigation by the department's internal affairs unit, Chinn said.

Officer William VanAntwerp, and Gunner, his partner for four years, were pursuing a man in connection with a traffic stop involving possible drunken driving. They had followed him to a house in the 1900 block of Winslow Court.

As the suspect ran into the house, VanAntwerp released Gunner from his squad car. VanAntwerp raced toward the front door, assuming that the dog was right behind him. But as he entered the house, the door closed, trapping Gunner outside, Chinn said.

It wasn't until he heard a gunshot and looked around to find Gunner missing that VanAntwerp realized something was wrong, Chinn said.

The officer who fatally wounded Gunner told police that the dog approached him aggressively and refused to let him enter the house where VanAntwerp was searching for the suspect. The officer, a member of the force for three years, said he did not know Gunner was a police dog.

"The dog approached the officer. The officer shot and killed the dog," Chinn said.

After hearing the gunshot, VanAntwerp ran outside and saw that Gunner had been wounded. He picked up the dog and took him to the Woodbridge Animal Hospital, but the animal died, Chinn said.

The dog's death has hit VanAntwerp hard, Chinn said.

Gunner, a dog trained to search for drugs and apprehend suspects, had been on the force for six years, Chinn said. The highly decorated dog had placed in many competitions and was recognized in one contest, sponsored by the U.S. Police Canine Association, as the best overall dog. He also had received awards for obedience and criminal apprehension.

The department will honor Gunner at a service, and he will be buried at a cemetery on police academy grounds reserved for police dogs.

"This is a sad day for us here," Chinn said.

Jason Beer, 24, of Woodbridge, the man officers were chasing Monday night, had been spotted by officers on Route 1 after he ran a stop sign, police said. Officers pulled Beer over, but he sped away and led the officers on a chase to his house. Beer then jumped out of the car and ran inside, police said.

Beer has been charged with driving under the influence, possession with intent to distribute marijuana, possession of a concealed weapon and eluding police, authorities said.

Prince William County police officer William VanAntwerp and his partner, Gunner. Gunner was mistakenly shot by another officer Monday night.