Authorities say they've found no sign of a reported gunman at the Naval Medical Center San Diego after a search that locked down the hospital for six hours.

The Navy says a room-to-room search began after an anonymous call came in around 7:30 a.m. Thursday.

Staff sheltered in place and hundreds of employees were unable to get to work until the search ended around 1:30 p.m.

U-T San Diego (http://bit.ly/1MojauN ) says investigators found four unspent bullets in the hospital but no shots had been fired. There's no word on who left the bullets or how long they had been there.

In January, someone reported gunshots at the hospital but authorities found nothing.

There also have been a dozen bomb threat hoaxes at local Navy bases and shipyards since November.