By Marty Graham

SAN DIEGO (Reuters) - A report of an active shooter and shots fired at the U.S. Naval Medical Center in San Diego triggered a heavy law enforcement response on Tuesday but was apparently unfounded, Navy officials said.

A lone witness reported hearing three shots at about 8 a.m. PST (1600 GMT) in the basement of a building at the sprawling medical center, the officials said.

Police interviewed the witness and continued to clear the building into Tuesday afternoon but there was no sign of foul play.

"We did an initial clearing of the building and found nothing that would indicate any kind of dangerous situation,” Navy Captain Curt Jones, commanding officer of the Naval facility, told a news conference.

California and the United States remain on edge after a married couple inspired by Islamist militants shot and killed 14 people last month in San Bernardino, about 100 miles (160 km) north of San Diego.

The city is home to the U.S. Navy's Pacific Fleet.

Seven law enforcement agencies, including U.S. Marshals, SWAT teams and an armored car from the San Diego County Sheriff's Department were involved in the response to the reported shooting.

Police with rifles and dogs searched the building where the shots were reported, as workers filed out with their hands in the air.

"As of right now, we have found absolutely nothing that indicates there were any shots fired," Jones said.

A post on the medical center's Facebook page had earlier advised occupants to "run, hide or fight," the protocol advised by the U.S. Department of Homeland Security in response to an active shooter.

All non-emergency response staff were asked to stay away from the compound. The shooter was reported in building #26, which is home to a "wounded warrior" program to help seriously injured and ill Navy sailors and members of the U.S. Coast Guard.

The medical center, about two miles (3 km) north of downtown San Diego, includes a 272-bed hospital and a staff of more than 6,500 military and civilian professionals, the center's website said.

A local middle school and two high schools in the area were initially placed on lockdown as a precaution.

(Reporting by Marty Graham in San Diego Additional reporting by Suzannah Gonzales in Chicago, Alex Dobuzinskis and Steve Gorman in Los Angeles and David Alexander in Washington; Editing by Tom Brown)