The Napa County Sheriff's Office confirmed police presence through an alert shortly before 11 a.m. Friday.
"We have activated our emergency response protocol and are cooperating with law enforcement", the California Department of Veterans said in a statement.
Napa County Fire captain Chase Beckman says a gunman has taken hostages at a veterans home in California.
"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.
The Napa Valley Register reported an armed gunman, dressed in black with body armor, entered the building and may have taken hostages.
Earlier, it emerged the veterans home was on lockdown after reports of shots fired.
Authorities are asking people to stay away from the Veterans Home in Yountville. About 1,000 veterans from conflicts spanning from World War II to Operation Enduring Freedom are said to call the facility home.
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.
The Yountville Home also serves the larger Napa Valley community with a 1,200-seat Lincoln Theater, where the Napa Valley Symphony plays, a 9-hole golf course, Recreational Vehicle park, the Borman Field baseball stadium, bowling lanes, swimming pool, and a military Base Exchange branch store.