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President George W. Bush talks with staff at the Naval Medical Center in San Diego in 2005. An active shooter was reported at the center Tuesday. Reuters

UPDATE: 1:15 p.m. EDT — Reports of an active shooter at Naval Medical Center in San Diego might have been a false alarm, officials said. Authorities "have not located any casualties or evidence of a shooting having taken place," Navy Region Southwest spokesman Scott Sutherland told Navy Times. "They are conducting a secondary, more thorough floor-by-floor sweep now."

UPDATE: 12:15 p.m. EDT — An active shooter fired at least three shots at the Naval Medical Center in San Diego Tuesday morning, witnesses told local reporters. The huge military hospital serves a large number of military personnel, retirees and family members, but no injuries have been reported.

UPDATE: 12:05 p.m. EDT — Public schools near the Naval Medical Center in San Diego have been put on lockdown amid reports of an active shooter at the hospital. The Navy center was also on lockdown.

Original story: The Naval Medical Center in San Diego warned staff members to run for cover Tuesday amid reports of an active shooter. The hospital posted on its Facebook page that the shooter had been reported in a building at the facility.

The message read: “**!ATTENTION!** 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, located at 34800 Bob Wilson Drive, San Diego, CA 92134.”

The San Diego Police Department was informed of the shooter, but was not asked for its assistance, a spokesman said. “They told us when they called in that we’re not needed at this time,” Officer Travis Easter, a San Diego police spokesman, told the Washington Post.

The medical center has a staff of more than 6,500 people and spans 1.2 million square feet in the city’s Balboa Park.

On its website, Naval Medical Center San Diego bills itself as "the largest and most comprehensive military healthcare system in the Western United States." NMCSD personnel work in Iraq, Afghanistan, Kuwait, Djibouti and aboard the USNS Mercy. The facility trains more than 400 students per year in clinical rotations, NBC News reported.

It's unclear whether the shooter planned to attack the center. Mass shootings have become common in the U.S. in recent years amid a national debate over gun control.