Tom Vanden Brook

USA TODAY

A lockdown at a Naval hospital in San Diego ended Thursday after authorities spent most of the day searching for a possible shooter who was not found, a Defense official told USA TODAY.

The search started Thursday morning after an anonymous caller reported an active shooter inside a building with other gunmen possibly on the way, the official said.

Law enforcement teams with dogs converged on Building One at the medical complex, the official said. They searched the building Thursday afternoon and recovered four unspent rounds of ammunition. There have been no injuries reported nor confirmation of shots fired.

The call matched another report of a patient seen with a weapon, according to the official, who was not authorized to speak publicly. All personnel were ordered to shelter in place as police responded to the scene. A child care center was also locked down and traffic diverted from the complex.

A similar report of an active shooter that turned out to be false came at the same naval hospital in January.

Officials also scoured buildings and grounds with dogs in January but found no evidence of a gunman or shots having been fired.

The medical center is located on the grounds of Balboa Park in San Diego. It is an advanced treatment facility and a major teaching and research center. About 6,000 people work there.

The initial report came from a caller who reported a patient with a gun and then hung up.