The fifth victim was identified as Oshiana Tompkins, 19, of Vallejo/Hercules, the Contra Costa County Sheriff’s Office said on its official Facebook page.
The sheriff’s office identified the names of the four other shooting victims earlier, including Tiyon Farley, 22, of Antioch; Omar Taylor, 24, of Pittsburg; Ramon Hill Jr., of San Francisco/Oakland; and Javin County, 29, of Sausalito/Richmond.
More than 100 people attended the Halloween event, which was advertised on social media, the post said.
Three people were pronounced dead at the scene and Tompkins was the second death declared at the hospital.
Several other people were injured and were transported to the hospital, officials said.
“There was a a lot of noise and yelling and people running,” Orinda police Chief David Cook said.
The Contra Costa district attorney’s office, a number of law enforcement agencies from the Bay area, and the FBI carried out a joint investigation.
“Two firearms were located in the house,” the sheriff’s office post said. “The crime lab is analyzing the weapons to determine if they were used in the shooting or any other crimes.”
No suspects have been arrested and the motive or if there was more than one shooter is unknown.