Bruce Pardo shot himself after the massacre in Covina. He arrived at a Christmas party at the home of Joseph and Alicia Ortega dressed as Santa Claus.
Covina Police Lt. Pat Buchanan said that Pardo expected his mother to be at the party, but she stayed home because she was sick, the Los Angeles Times reported. He left a rented car outside the lawyer's house, although police said they have not determined what he planned to do with the vehicle.
The city held a community meeting Monday night to discuss the killings with local residents.
Mayor Kevin Stapleton promised the 500 people that the killings are "not going to define Covina." He also urged outsiders to allow residents and especially relatives of the dead time to get some "sense of normalcy," the Times reported.
Police, updating residents on the investigation, said that Pardo began buying guns in June, suggesting that he had been planning a violent act for at least six months.