Photo: The Canadian Press

A San Antonio, Texas, police officer writing out a traffic ticket to a motorist was shot to death in his squad car Sunday outside police headquarters by another driver who pulled up from behind, authorities said.

San Antonio police Chief William McManus identified the officer as Detective Benjamin Marconi, 50, a 20-year veteran of the force.

McManus said the suspect had not been apprehended Sunday night. He said he doesn't believe the man has any relationship to the original motorist who was pulled over, and no motive has been identified.

"We consider this suspect to be extremely dangerous and a clear threat to law enforcement officers and the public," said McManus, who added that after the shooting officers had been instructed to not make traffic stops alone.

McManus said Marconi had pulled over a vehicle and while he was inside his squad car writing a ticket, a car pulled up behind him. The driver of that car got out, walked up to the officer's driver-side window and shot Marconi twice in the head, then walked back to his car and drove away.

Marconi was pronounced dead at a hospital.

McManus said investigators are looking into all leads and motives, including whether it could be related to an officer-involved shooting earlier Sunday in the city. In that incident, McManus said, police fatally shot a man who pointed a gun at officers outside an apartment following a seven-hour standoff.