The law enforcement departments who have responded to the hostage situation include the Napa County Sheriff, Napa Police Department, Sonoma County Sheriff, California Highway Patrol and the Federal Bureau of Investigation.
Kamer said he does not know why the shooter let his wife and some colleagues leave the party.
The Napa Vally Register says the gunman is holding two people hostage.
The California Department of Veterans Affairs said it was following reports of gunfire at the home, and that the safety of its residents, workers and the community were its top priority.
Robertson said about 70 students who were inside the Lincoln Theater, located on the grounds of the veterans home, have been evacuated and are safe. Authorities know his identity, he said, though it has not been disclosed yet. The program treats veterans of the Iraq and Afghanistan wars with post-traumatic stress disorder.
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Officials say they have yet to make contact with the suspect, but hostage negotiations will happen when they do. He didn't give a motive for the gunman's actions.
California Highway Patrol Assistant Chief Chris Childs says they haven't made contact with the gunman who's confined to one room but that hostage negotiators are standing by.
Jan Thornton of Vallejo, California, was among hundreds of relatives anxious how their loved ones were coping with the lockdown.
Thornton says her 96-year-old father - a WWII fighter pilot - is inside a hospital wing at the home in Yountville, north of San Francisco.
Events manager Elizabeth Naylor, who was working about 10 miles north of the veterans' home, said she heard waves of emergency sirens. CHP and Napa County fire officials could not immediately be reached for comment.
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Founded in 1884, the facility houses about 1,000 aging vets and is the largest veterans home in the United States, according to the CalVet.
"We do have an active shooter situation with a hostage situation in Yountville", Highway Patrol Officer John Fransen told San Francisco Bay Area news station KTVU-TV.
According to California Department of Veterans Affairs, the Veterans Home of California in Yountville dates back to 1884 and is considered the largest veterans home in the USA with more than 1000 veterans from all wars dating back to World War II.
Police closed access to the large veterans home in Yountville after a man with a gun was reported on the grounds.
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