A fender-bender in Prince George's County last night turned into a gruesome scene with two deaths after one driver fatally beat the other on the street, and an off-duty D.C. police officer passing by shot the man who threw the lethal punches, police said.

The violence unfolded about 7:45 p.m. along a stretch of Marlboro Pike in Capitol Heights, when a man driving a red Chevrolet pickup truck struck the back of a black Chevy SUV. Witnesses told police that the pickup continued to ram the SUV, pushing it for several blocks, long after the SUV's engine had stopped running, said Prince George's County police spokeswoman Cpl. Diane Richardson.

At Capitol Heights Boulevard, in front of a dentist's office, a community center and a cemetery, the man in the pickup got out and confronted the driver of the other vehicle. In the middle of busy Marlboro Pike, he began beating the tiny man, an assault that witnesses described as "vicious," Richardson said.

An off-duty D.C. police officer driving home happened upon the scene. He told investigators that the smaller man already was unconscious and that it looked as though the pickup driver was beating a lifeless body, she said.

The officer identified himself and told the attacker to stop, Richardson said. But the man continued the assault and then charged at the officer, who shot the man at least once in the chest, she said.

That man was taken to Prince George's Hospital Center, where he died from the gunshot wound shortly after arrival, she said. The man who was beaten died in the middle of the road.

The D.C. police department's Force Investigation Team was called to investigate the officer's use of his weapon, but they won't start that part of the case until the Prince George's County detectives are finished, said Sgt. Joe Gentile, a D.C. police spokesman.

Police identified neither the officer nor the motorists last night.