The incident happened in the 7300 block of Palmetto Avenue. The bodies were discovered by a 16-year-old boy who was concerned that he hadn't heard from his mother.
When police arrived, they found the bodies of Ramon Miranda, 38; Silvia Miranda, 34; Rayna Miranda, 10; and Ramon Miranda Jr., 12. They were confirmed deceased at 8:38 p.m.
Investigators say the woman and children died of multiple gunshot wounds, and the man died of one gunshot wound to the head.
Though the investigation is in the early stages, Fontana Police spokeswoman Martha Guzman-Hurtado says the incident appears to be a murder-suicide. Police say they recovered a handgun next to the man's body.
Neighbors told Eyewitness News that a mother, stepfather and two children live in the home.
"Everything that has been confirmed so far indicates it could be a murder-suicide but that has not been confirmed," said Guzman-Hurtado. "They are questioning other people before making that conclusion."
Fontana Police Sgt. Doug Imhof described the grisly scene at the home.
"There is some amounts of blood in different areas of the house, yes. There's a couple of weapons that have been seen by our detectives just looking, but again, they haven't been able to search the house because they can't until they obtain the search warrant," he said.
There was no history of domestic violence problems at the home, but neighbors say there had been arguing in the house earlier in the evening.
"I was talking to the neighbors and they actually told me that they heard the couple arguing before any of this happened," said neighbor Jessica Arauz.
Police say they are not looking for any outstanding suspects.
A motive for the crime remains unknown, and the exact relationship between the man, woman, and two children has not been confirmed, according to police.
The Associated Press contributed to this report.