Jockey Jorge Herrera was killed when he was thrown from his mount in the eighth race July 5 at the Alameda County Fair in Pleasanton, Calif., a racing official confirmed.

Herrera, 33, suffered a massive head injury when he either struck the ground or was clipped by another horse, according to the official, who asked not to be identified while trying to piece together exactly what happened. After the accident, Herrera was taken to a nearby hospital, where he died less than two hours later.

According to the Equibase race chart, Morito, the horse Herrera was riding in the $5,000 maiden claiming race at five furlongs, came out slightly while racing between horses leaving the half-mile pole. The 4-year-old gelding clipped heels with another horse, Tribal Sun, and stumbled badly, throwing Herrera.

Herrera, from Jalisco, Mexico, was an exercise rider and journeyman jockey who had raced in California, Oregon, and Washington. Herrera won 55 races during his career from 1,008 mounts. He had raced sparingly in the past 2 1/2 years after getting his start in 2004.

A statement from the Alameda County Fair said: "As is our normal safety protocol, an ambulance was following seconds behind the race, therefore, emergency medical personnel responded immediately. Mr. Herrera was transported to Eden Trauma Center in Castro Valley in very critical condition where he later passed away. We are deeply saddened by this loss, and our thoughts and prayers are with his family."

Herrera was the first jockey to die at Pleasanton since leading Northern California rider Juan Gonzalez was killed in 1975.