The three women were all employed by Pathway Home, Chris Childs, California Highway Patrol (CHP) assistant chief, said in a statement Friday evening.
Pathway Home is a non-profit organization located at the Veterans Home of California, Yountville, in Napa County, about 85 km north of San Francisco. It runs a program that treats US veterans of the Iraq and Afghanistan wars with post-traumatic stress disorder.
"This is a tragic piece of news, one that we were really hoping we would not have to come before the public to give," Childs said.
The three victims were identified as Pathway Home Executive Director Christine Loeber, 48; Clinical Director Jennifer Golick, 42; and Jennifer Gonzales, 29, a clinical psychologist with the San Francisco Department of Veterans Affairs Healthcare System.
The suspected gunman was identified as Albert Wong, a former US soldier who served on active duty from May 2010 to August 2013 and spent a year in Afghanistan.
On Friday morning, the 36-year-old, dressed in black body armor and armed with an automatic rifle, entered the center during a going-away party for some employees, and took the three women hostage.
Four gunshots were heard from the when some 80 students were at a theater rehearsing a play "a distance away from the shooting situation," Napa County Sheriff John Robertson said, adding that the teens were safely evacuated.
Childs said it was a deputy from Napa County sheriff's office that exchanged gunfire with the gunman that eliminated "the ability for the suspect to go out and find further victims," after which the police failed to make contact with the gunman.
"Shortly before 6 p.m. (0200 GMT, Saturday) this evening, law enforcement personal made entry into the room where the hostages were being held by the suspect and unfortunately made the discovery of three deceased females and one deceased male (suspect)," Childs said.
"These brave women were accomplished professionals who dedicated their careers to serving our nation's veterans, working closely with those in the greatest need of attention after deployments in Iraq and Afghanistan," Pathway Home said in a statement.
Jennifer Golick's father-in-law said in an interview that she had recently expelled Wong from the program. Childs did not confirm whether the hostages were chosen at random or were targeted, since investigators had not yet determined a motive.
The gunman's rented vehicle was later discovered in the parking lot. The CHP special police team and the Explosive Ordnance Disposal Unit were called in to search for possible bombs and found nothing. They cleared the car, which posed no threat to public safety, Childs said.
The Yountville home, which opened in 1884 and is the nation's largest veterans' home, houses about 1,000 elderly and disabled residents, according to the California Department of Veterans Affairs.