SWAT teams have swept a naval hospital in San Diego after a suspected gunman reportedly fired shots into the air and found no signs of a shooting or a shooter.

Tactical units started searching the building and placed it on lockdown after a witness reported hearing three shots coming from the basement.

Naval Medical Center San Diego posted on its Facebook account a little after 8am on Tuesday; '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.'

The Naval Medical Center said that the initial sweep of the building could not substantiate the report of a shooting, but they are still investigating and the FBI is on the scene.

Among those who were assisting were U.S. marshals, the Chula Vista Police Department and the California Highway Patrol.

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Naval Medical Center San Diego (above) said there was a report of an active shooter at the facility

San Diego Police confirmed reports of a shooter and SWAT and FBI were both seen arriving on the scene to investigate

Military personnel were evacuated with their hands raised at the Naval Medical Center in San Diego during the first sweep

'The investigation of Building 26 continues. First responders and Navy working dog units have conducted an initial top down inspection of the building and have not located any casualties or evidence of a shooting having taken place,' said Navy officials.

'There have been no reported casualties at this time. There is a secondary, more thorough floor by floor inspection taking place at this time.

'The medical facility remains on lockdown and a shelter in place order remains in effect.'

Navy spokesman Jon Nylander had said earlier in the day; 'Apparently someone said they heard three shots in the basement of Building 26. Maybe they did or not is not the point; that’s what they reported, so that’s why we responded accordingly.

'We haven’t gotten any reports of any injuries.'

The Facebook post went up a little after 8am on Tuesday morning

A San Diego Police SWAT vehicle arrives outside of the Naval Medical Center San Diego

Captain Curt Jones, commanding officer of Naval Base San Diego, spoke to the media after the first sweep of the building, saying: 'We have done a number of clearing sweeps of the building and as of this time, we have found nothing that substantiates those reports.

'We are continuing to clear the building to ensure there are no casualties and there’s nothing that’s not quite right in the building.

'I can tell you the person who reported the gunshots was in the building at the time and reported that they thought they heard three gunshots.'

He then added: 'As of now we have found absolutely nothing that indicates there were any shots fired. There are no casualties at this time.'

When asked to give information about the person who reported the incident he said he refused to identify the individual.

People are evacuated from a building at the Naval Medical Center after shots were reportedly heard

Capt. Curt Jones, Commander of Naval Base San Diego, speaks to members of the media during a news conference Tuesday

The 272-bed, multispeciality hospital and ambulatory complex is located on about 78 acres in the southeast corner of Balboa Park, about 2.5 miles from downtown, according to its website. Center staff is comprised of more than 6,500 military, civilian, contractor and volunteer personnel.

NMCSD personnel deploy to Iraq, Afghanistan, Kuwait, Djibouti and aboard the USNS Mercy, the website said.