Michael Pimentel was gunned down just before noon near South First Avenue and East Ninth Street, where he conducted the traffic stop, Bexar Sheriff Susan Pamerleau said.
Pamerleau said the suspect, identified as Joshua Lopez, 24, is in custody. He is to face a charge of capital murder of a police officer. Pamerleau said the Bexar County Sheriff's Office has taken over the investigation of the incident.
Details surrounding the traffic stop were not disclosed, but there was an active graffiti warrant out for Lopez, Pamerleau added.
Pimentel was shot twice, once in the shoulder and once in abdomen, said Sgt. Jason Burchett, who's now the Elmendorf's acting police chief. He added that the suspect suffered from mental illness.
Alderman Polo Maldonado and Alderwoman Linda Pena Ortiz confirmed the chief, who was flown to University Hospital, had died from his injuries.