San Diego's Naval hospital was placed on lockdown Thursday following a report of an armed man in one of the buildings, authorities said.
Hospital officials said the alarm was raised after someone spotted the man on the fourth floor of the hospital's main building and made an anonymous call to police.
"As a measure of precaution staff are sheltering in place -- staff and patients en route to NMCSD are advised not to come to the hospital until the scene is cleared," the hospital said in a message on its Twitter account.
Law enforcement rushed to the hospital, also known as Balboa hospital, with canine units and were sweeping the buildings floor by floor.
There were unconfirmed reports that the person spotted with a gun was a patient.
It is the second such security scare at the facility in as many months.
In January, the hospital was placed on lockdown following what proved to be a false report of an active shooter.