Travis Air Force Base in Fairfield responded to what was being described as a 'real world security incident.'
The base was scheduled to conduct an exercise on Wednesday and Thursday simulating a "security incident" before the alert happened.
The base was shutdown and urged people to shelter in place, lock doors and windows, according to their Twitter page. The public was being asked to "stay away from the base to ensure emergency responders can respond accordingly" before the lockdown was eventually lifted.
The biggest incident occurred in 2014 when a pilot was killed by fire after his plane crashed during the Travis AFB Airshow. The autopsy report indicated 77-year-old pilot Eddie Andreini survived the crash, but was then burned to death.
Last year on May 3, Travis AFB was on lockdown in the morning, but later that day representatives at the base confirmed it was due to a training exercise.