A police officer was apparently assassinated during a traffic stop on Sunday, San Antonio police said.

San Antonio Police Chief William P. McManus said Sunday that the officer was sitting inside his car just outside police headquarters when he was shot and killed.

The officer —  beyond the fact that he was a 20-year veteran — has not been identified in the shooting that happened around 11:40 a.m. on Sunday, reports CBS San Antonio affiliate KENS.

McManus said the officer had pulled someone over and was writing them a ticket when a black car pulled up behind the officer and stopped.