Authorities say a San Antonio police officer has been fatally shot while writing a traffic ticket outside of police headquarters.

San Antonio police Chief William McManus said the officer was killed late Sunday morning. He says the suspect has not yet been apprehended and a motive is not known.

McManus says the officer had pulled over a vehicle and while he was inside his vehicle writing a ticket, a vehicle pulled up behind him. He says the driver of that vehicle got out, walked up to the officer's driver-side window and shot the officer in the head.

The officer was pronounced dead at a hospital. He was a 20-year veteran of the police force.

McManus says, "This is everyone's worst nightmare."