The Contra Costa County Office confirmed the news on Friday, after saying it had an "active investigation" at the Orinda home that is around 30km north of San Francisco. The party took place at Orinda Hill, North California in front of a quiet street.
In the Orinda shooting, one of the people wounded was in critical condition at the John Muir Medical Center in the nearby city of Walnut Creek, hospital spokesman Ben Drew said.
He says he saw more than a dozen people there using footage from the Ring doorbell camera at his house.
No information on the victims was immediately available and police were investigating what prompted the violence. Video from the scene showed multiple patients being loaded into ambulances as others limped away from the scene.
"Our whole neighbourhood heard it and then people were screaming and fleeing", Chris Gade, who lives near the house, told reporters, the San Francisco Chronicle reported. "This just doesn't happen in Orinda".
"It's a very sombre day here in Orinda", mayor Inga Miller told local radio. The maximum occupancy is two people per bedroom plus three people not assigned to bedrooms.