Travis Air Force Base northeast of San Francisco is locked down due to what the base calls a security incident.
The base issued a security alert shortly after 3:30 p.m. Wednesday asking people to shelter in place and to lock doors and windows.
Marie Lowe, a civilian who answered the phone at the base's public affairs office, says details are sketchy and officials are investigating because they don't know exactly what is going on.
Base officials asked on Facebook for the public to stay away to ensure emergency responders can do their work.
The lockdown comes on the same day that the base had been conducting security lockdown exercises, but officials say the incident is real and not part of the drill.