Authorities responded to the 1000 block of South Magnolia Avenue, near Mission Boulevard, after receiving a call about a person lying in the driveway of a house at 6:15 a.m. When officers arrived, they found a body in the driveway between a front and rear house at the location. The victim appeared to have suffered a gunshot wound.
When police entered the rear house, two more men were found dead. It was unclear what injuries they suffered.
The names of the deceased have not yet been released. Police say they appear to be in their mid-30s. Investigators say at this point, it is unknown if the men were victims, suspects or acquaintances.
Ruben Jacquez, who lives nearby, said people often shoot fireworks in the area, and he thought he heard fireworks or gunshots around 2 a.m.
"We heard four to five pops. It was silent for a minute, and in that silence, we heard someone scream, and then we heard another pop," he said. "Me and my dad went out, we didn't see anybody."
Neighbors said they saw two men walking away from the scene sometime after the shots were heard. But police said it's still too early to tell if they were suspects. Investigators carried out bags of evidence including shotguns, rifles and marijuana.
Ontario Police Sgt. David McBride said there are no indications that it was a murder-suicide. Officials believe there are suspects still outstanding.
Anyone with information is urged to call the Ontario Police Department at (909) 395-2764.