Sheriff's detectives believe was targeted in a car-to-car attack by rival gang members.
Guillermo Reyes and detectives determined the shooting occurred on Center Drive near Los Angeles street about 10 p.m. that Monday night.
A passenger in the car Reyes had been driving, 21-year-old Jamie Martinez, was arrested on suspicion of auto theft and illegally carrying a loaded gun, a sheriff's detective told the San Gabriel Valley Tribune.
Martinez drove Reyes to a hospital. He pleaded not guilty June 28 to felony charges of carrying a loaded and unregistered handgun, carrying a loaded firearm with intent to commit a felony and auto theft.
Prosecutors allege the auto theft was done for the benefit of a criminal street gang, a sentencing enhancement that requires prison time if convicted.
On July 12, Martinez is due in West Covina Superior Court for a preliminary hearing, the newspaper reported.