Officials at Travis Air Force Base in California have advised people there to "shelter in place" during a "real world security incident," according to official social media posts issued on Wednesday afternoon.
"Travis Air Force Base is currently responding to a real world security incident. More details will be released as they become available. The public is being asked to stay away from the base to ensure emergency responders can respond accordingly," said a post on the base's Facebook page.
The base's official Twitter account advised people at there to lock their doors and windows and await further information.