Seventeen-year veteran Francisco Gamez, 41, was taken into custody following a months-long Los Angeles Police Department investigation into the death of Armando Casillas.
"The off-duty acts that led to the arrest of Francisco Gamez are deeply disturbing to me and to the entire Los Angeles County Sheriff's Department," Sheriff Lee Baca said in a statement Wednesday.
Just after 11 a.m. June 17, patrol officers were flagged down in Sylmar after Casillas was gunned down. He was pronounced dead after being taken to Providence Holy Cross Hospital.
The assailant also shot at a second victim who was not injured.
LAPD Officer Venus Hall said details about the nature of the incident were not being released.
Gamez was being held on suspicion of murder, attempted murder and use of a firearm during the commission of a felony. The district attorney's office had not yet received a filing from LAPD detectives requesting formal charges.
Gamez was being held in lieu of $4 million bail at the 77th Street Jail. Neither the sheriff's office nor LAPD knew if he had a lawyer who could be reached for comment.
Gamez has been assigned to the West Hollywood station since 2005. He has been relieved of duty with pay since July 3. Sheriff's spokesman Steve Whitmore said Wednesday that proceedings would begin Wednesday to stop Gamez's pay, and termination was likely.