The officer found his elderly mother inside her South Los Angeles home around noon, department spokesman Cleon Joseph said.
He said police had no suspect information and did not know the motive for the attack.
A family friend said the officer and his mother were very close.
"He checks up on his mother every day," Allyson Smith said.
"She was beautiful, I mean a lovely mother," Smith said. "That's her only son."
KABC-TV reported that the officer is a department veteran who trains recruits at the police academy. He stayed at the crime scene while his colleagues conducted their investigation.
"A lot of his coworkers are here. He's really grieving and doesn't want to leave the scene until the coroners come get his mother's body," Capt. Dennis Kato told the news station.