The shooting Thursday night in Orinda, near Oakland, happened at a party attended by 100 people, police chief David Cook told reporters during a press conference Friday. The shooting occurred around 11 p.m. PST.
A stretcher flanked by authorities outside the Californian home where the shooting took place during a Halloween party.
The Contra Costa County Sheriff's Office announced the casualties in a tweet Friday.
Video footage broadcast by ABC7 News showed a person on a stretcher being loaded into an ambulance and young people who appeared to be party guests, including some in costumes, walking in the street outside the house. They found a party with around 100 people there.
Michael Wang, who owns the 4,000-square-foot (372-square-meter) house, said he rented it out to a woman through Airbnb for what the woman said would be a family reunion for a dozen people, according to The San Francisco Chronicle.
Wang says he got calls from the neighbors about the noise and that his wife reached out to the woman, who claimed there were only 12 people in the home.
The home where the party was held is in a neighborhood of multimillion-dollar homes.
Airbnb's head of trust and safety said it was "urgently investigating" what happened. The maximum occupancy is two people per bedroom plus three people not assigned to bedrooms.