SAN FRANCISCO -- Three female hostages and a suspected gunman were found dead after police entered a veterans' home in Yountville in the US state of California, local police said late Friday.

It appeared that the gunman killed himself, law enforcement officials said, adding that the names of the dead could not be released by the Coroner's Office of Napa County, about 85 km north of San Francisco.

"This is a tragic piece of news, one that we were really hoping we would not have to come before the public to give," said California Highway Patrol Assistant Chief Chris Childs.

The bodies of the four were discovered nearly eight hours after the gunman entered the Veterans Home of California in Yountville, which housed Pathway Home, a non-profit organization running a program that treats U.S. veterans of the Iraq and Afghanistan wars with post-traumatic stress disorder.

Childs could not confirm whether the hostages were chosen at random or were targeted.

He said police have searched the gunman's rented vehicle in the parking lot for bombs, but ultimately found nothing.

Local media quoted police sources as saying that at least one police officer had exchanged fire with the suspect, but no one was injured in the shooting.

The three females were employees of Pathway Home, who were taken hostage shortly after the 36-year-old gunman, who was dressed in black armor and was armed with an automatic weapon, broke into the veterans' home Friday morning.