A security alert was issued at the US Naval Medical Center in San Diego on Thursday after a gunman was reported to have been seen inside the main hospital, but the alert was lifted hours later when no suspect was found, a base spokesman said.
The incident began when the Navy complex posted an advisory on its Facebook page saying an "active shooter" was present at the hospital and advised all personnel and their families to avoid the area, though no gunfire was reported.
Base spokesman Michael Wiener said military security searched for a possible suspect but did not find anyone after seven hours.
An anonymous caller reported seeing an unidentified man with a firearm on the fourth floor of the hospital's west wing, a nursing ward, prompting shelter-in-place orders to be issued for the hospital, Wiener said.
Four unspent bullets were found during the search, but it was not immediately clear if those were related to the telephone call that prompted the alert, he said.
The medical center, about 2 miles (3 km) north of downtown San Diego, includes a 272-bed hospital and staff of more than 6,500 military and civilian professionals. San Diego is home to the U.S. Navy's Pacific Fleet.