The four bodies were found late Thursday inside the single-story house in a residential area of Pomona, police Lt. Michael Keltner said at a morning news conference.
The suspect lived at the home, and the murder-suicide appears to have stemmed from a domestic situation, Keltner said. A woman in her 50s and her 24-year-old daughter and 17-year-old son were killed in their respective bedrooms, he said.
The suspect was found at the foot of the younger woman's bed. Keltner said she might have been his girlfriend. There were no signs of a struggle.
Their names were withheld pending notification of their relatives.
Officers went to the house after the man called relatives of the victims and said he was going to harm the family, police said.
Leonardo Pina, 38, lives down the street and described hearing gunshots around 11 p.m. He said that while the neighborhood is accustomed to violent crime, the apparent murder-suicide was a jolt to residents who described the family as being quiet but friendly.
"We're used to drug stuff and gang crimes, but not a murder-suicide," Pina told the Los Angeles Times. "This is something different. It's horrible."
Investigators were still trying to determine a motive for the shootings, Keltner said.
Pomona is a city of about 150,000 people 30 miles east of downtown Los Angeles.