Authorities have not found evidence that a gun was fired at Naval Medical Center San Diego after a witness reported hearing three shots in a building there.
"We've done an initial sweep and ... have found nothing," U.S. Navy Captain Curt Jones told reporters Tuesday. "We're on a secondary sweep now."
The federal health facility went into lockdown Tuesday morning after a person reportedly heard shots go off around 8 a.m. PT in the basement of Building 26, which houses a gym and barracks. Jones said authorities were "investigating now as to what might have caused" the sound.
"There is nothing that we can discover that has occurred," he said.
Earlier, the sprawling health facility, which employs more than 6,500 military, civilian, contractor and volunteer personnel, said on Facebook that there had been reports of an "active shooter."
"An active shooter has just been been reported in building #26 at Naval Medical Center San Diego. All occupants are advised to run, hide or fight. All non-emergency response personnel are asked to stay away from the compound," it posted.
The complex was "secured," the medical center later posted, adding that all children in childcare facilities at the medical center were "accounted for and safe." There were no injuries reported.
Nearby elementary and middle schools, as well as Naval Base Point Loma, also in San Diego, were also briefly placed on precautionary lockdown, NBC affiliate KNSD reported.
Jones praised the response to the incident.
"We take all reports like this very seriously. We do a lot of training to help people recognize what they're supposed to do in the event of an active shooter," he said.
Located about 2 1/2 miles from downtown San Diego, the 272-bed hospital has personnel who deploy to Iraq, Afghanistan, Kuwait, Djibouti and aboard the USNS Mercy, a Navy hospital ship.
This article first appeared on NBCNews.com.