Officers responded to reports of a shooting in the area of Euclid Avenue and 6th Street around 8:30 p.m., Ontario police Cpl. Eliseo Guerrero said.
Two males were found at the location: an off-duty LAPD officer and a man with a gunshot wound who was pronounced dead at the scene, Guerrero said. Neighbors said the deceased man was lying face-down on the ground. He was not immediately identified.
The off-duty LAPD officer was taken to a hospital in stable condition with unspecified injuries, and was listed in stable condition.
Jason Smith, who lives near the scene, said he was in his backyard when he heard "rapid fire -- bam, bam, bam, bam, bam."
It's unclear what led up to the shooting or if the officer was shot at. One witness said the incident may have started with a confrontation over a minor traffic crash.
"An old man came out -- put his (hazard lights) on -- the one who got hit," said Will Sasse. "They both continued to pull off to the side. At this point I was stopped by the red light. I saw two gentlemen came out, arms flailing."
The officer is cooperating with investigators, Guerrero said.
Nearby resident Genevieve Miranda said she "heard the gunshots, so I panicked. This is not an area where you normally hear gunshots."
The LAPD released a statement saying it "is aware of an officer involved shooting involving a Los Angeles police officer in the city of Ontario earlier this evening. The Los Angeles Police Department is fully cooperating with the investigation."