Three workers at the largest veterans home in the US have been found dead after they were taken hostage.

The bodies of the women and the hostage-taker were discovered nearly eight hours after the gunman slipped into an employee going-away party.

The victims worked for a programme that treats combat veterans of Iraq and Afghanistan for post-traumatic stress disorder, said California Highway Patrol Assistant Chief Chris Childs.

They were employees of the non-profit organisation Pathway Home treatment programme, which is housed on the campus of the Veterans Home of California-Yountville.

Napa County Sheriff John Robertson declined to identify the victims and the gunman on Friday night, saying family members still needed to be notified.

Authorities said they know who the gunman is but did not reveal his identity or know the motive for the attack at the state-run home.

Mr Childs said investigators have not determined a motive.

"It's far too early to say if they were chosen at random," Mr Childs said.

Yountville is about 53 miles north of San Francisco.

A sheriff's deputy responding to an emergency call shortly after 10am got into a shootout with the gunman, but the officer was not injured.

Highway Patrol Sgt Robert Nacke said negotiators were unable to make contact with the gunman throughout the day.

Larry Kamer told The Associated Press that his wife, Devereaux Smith, was at a morning staff party and told him by phone that the gunman had entered the room quietly, letting some people leave while taking others hostage.

Ms Smith, a fundraiser for the nonprofit Pathway Home, was still inside the facility's dining hall and was not allowed to leave, he said.

The Pathway Home, a privately run programme on the grounds of the veterans home, treats veterans of the Iraq and Afghanistan wars with post-traumatic stress disorder.

Police evacuated the property and closed off nearby roads. An armoured police vehicle, ambulances and several firetrucks were at the facility, which houses about 1,000 residents.

A group of about 80 students who were on the home's grounds were safely evacuated after being locked down, the sheriff said.

The teens from Justin-Siena High School were at a theatre rehearsing a play.

The state Veterans Affairs department says the home that opened in 1984 is the nation's largest veterans home, with about 1,000 elderly and disabled residents.