U.S. authorities are investigating a possible active shooting incident at the Naval Medical Center in the western city of San Diego, California, but say their initial search has not turned up any victims or a shooter.
The authorities reported that one witness heard three shots fired in the basement of Building 26 of the medical center, with the hospital advising people in the facility to "run, hide or fight."
Law enforcement authorities are now conducting a further search of the building.
The facility was locked down as authorities investigated. Building 26 houses a gym, as well as offices and barracks for wounded sailors and Marines.
During the past several years, there have been numerous shooting incidents in public places in the U.S., including one in December in which an American Muslim and his Pakistani wife killed 14 people at a government center in San Bernardino, California, before police killed them several hours later in a massive shootout.
Gun ownership in the U.S. is enshrined in the country's Constitution, but restrictions on the use of guns is proving to be a divisive political issue in the 2016 presidential campaign. Democratic presidential contenders are calling for stiffer regulations, while the Republican candidates have staunchly defended the rights of gun owners.
President Barack Obama recently issued an executive order calling for tighter background checks on gun buyers, an action that is being contested in a lawsuit.