The four-bedroom home had been rented on Airbnb by a woman who told the owner her dozen family members had asthma and needed to escape smoke from a wildfire, a person with knowledge of the transaction said.
The owner was suspicious of a one-night rental on Halloween and before agreeing reminded the renter that no parties were allowed.
In a series of tweets, Airbnb CEO Brian Chesky said Saturday the San Francisco-based company is expanding manual screening of “high risk” reservations and will remove guests who fail to comply with policies banning parties at Airbnb rental homes.
Mr Chesky also said the company is forming a “rapid response team” when complaints of unauthorised parties come in.
“We must do better, and we will. This is unacceptable,” he tweeted.
Before the shooting on Thursday night, the owner of the property, Michael Wang, said his wife reached out to the renter after neighbours contacted them about the party.
The renter said there were only a dozen people at the home but Mr Wang said he could see more people on video from his doorbell camera.
“We called the police. They were on the way to go there to stop them, but before we got there the neighbour already sent us a message saying there was a shooting,” he told the Chronicle.
No arrests had been made and there was no immediate word on a motive for the attack.
Two guns were found at the property, authorities said.
Three people, all from the Bay Area, died at the scene and a fourth died at the hospital, authorities initially said.
The Contra Costa County Sheriff’s Office identified them on Friday evening as Tiyon Farley, 22; Omar Taylor, 24; Ramon Hill Jr, 23; and Javin County, 29.
The sheriff’s office identified a fifth victim, 19-year-old Oshiana Tompkins, late on Friday night, saying she died at a hospital.
The party at the four-bedroom house was apparently advertised on social media as an “Airbnb mansion party”.
Additional reporting by Associated Press