Police closed access to the large veterans home in Yountville after a man with a gun was reported on the grounds.
The Napa County Sheriff's Department issued an alert to residents at 10:30 am (local time) warning them to avoid the area because of "activity at the Veterans Home in Yountville."
The Napa Valley Register reported that a man wearing body armour and armed with an automatic weapon entered the home.
The sheriff's department did not immediately respond to a telephone call from The Associated Press.
The state Veterans Affairs department says the home that opened in 1984 is the largest veterans' home in the United States, with about 1,000 elderly and disabled residents. Its website says it offers residential accommodations with recreational, social, and therapeutic activities for independent living.
Veterans of World War II, Korean War, Vietnam War, Desert Storm, and Operation Enduring Freedom/Operation Iraqi Freedom now live at the home, according to the website.
The grounds are also home to a 1,200-seat theater, a 9-hole golf course, a baseball stadium, bowling lanes, a swimming pool, and a military Base Exchange branch store.
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