Bay Area quoted a local fire captain as saying that a gunman had taken hostages inside the home.
Robert Nacke told reporters that he knew of no injuries, and authorities gave no immediate information about the gunman or his motive.
Jan Thornton of Vallejo is among hundreds of relatives anxious about their loved ones at a Northern California veterans home that was locked down after reports of an active shooter.
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 Veterans Home of California opened in 1884 and is the oldest veterans home in the country.
An armored police vehicle, ambulances and several firetrucks could be seen at the scene.
Fransen said authorities were working to establish a secure perimeter around the facility, which is home to about 1,000 residents.
A man wearing body armour and armed with an automatic weapon reportedly entered the
The county sheriff's office responded to a call at around 10:30 a.m. when the suspect allegedly walked into a party hosted by Pathway Home, a program that works with veterans who have post-traumatic stress disorder.
The armed suspect was dressed in black with body armor, according to local news. He said he did not know if the shooter was a veteran receiving treatment from the home.
The sheriff's department did not immediately respond to a telephone call from The Associated Press.
"Law enforcement is at the Yountsville Veteran's Home right now following reports of gunfire", they posted on Twitter.
"Everyone is just in alert mode, and we're taking information as it comes", state Department of Veterans Affairs spokesman Joshua Kiser told the Los Angeles Times in a phone interview from his office on the home's grounds. Its website says it offers residential accommodations with recreational, social, and therapeutic activities for independent living.
The grounds are also home to a 1,200-seat theater, a nine-hole golf course, a baseball stadium, bowling lanes, a swimming pool and a military Base Exchange branch store.
Larry Kramer told Reuters his wife worked at the facility, the largest of its kind in the country and home to about 1,000 veterans about 50 miles (80 km) northeast of San Francisco.
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