Neighbors reported hearing gunfire shortly before midnight Thursday. Police also got a call from a relative of the alleged shooter, who said he had threatened to harm his girlfriend and her family.
Officers who went to the house found the bodies early Friday.
The identities of the victims had not been released late Friday morning. Police said the mother was 52, her daughter 25, and her son 17.
Police Lt. Mike Keltner said at least two of the dead were probably awake when they were shot, but there was no sign they tried to escape or stop the killer. He said all four people could have been killed within 30 seconds.
Leonardo Pina, who lives nearby, told the Los Angeles Times that this kind of domestic slaughter is shocking in a neighborhood used to violent crime.
"We're used to drug stuff and gang crimes, but not a murder-suicide," Pina said. "This is something different, it's horrible."
Pomona, 27 miles east of Los Angeles, has grown from a farming community to a dense, ethnically-diverse city.