By Kristina Wong - 01/26/16 11:43 AM EST
A U.S. Navy medical center in San Diego on Tuesday morning reported that there was an active shooter in one of its buildings.
A single witness reported hearing three shots in the basement of Building 26, according to a Navy statement.
"First responders and Navy working dog units are on scene clearing Building 26. The Medical Center is on lockdown and a shelter in place order has been given," it said.
A floor-by-floor search of the building was being conducted, a Navy spokesman said.
The building houses a fitness unit, recreational center and dormitories for recovering troops.
About two hours after the alert, people began evacuating from inside the center, walking out with their hands in the air as they were patted down by officers.
There was no evidence yet to substantiate a single witness's report of shots being fired, a Navy commander said later.
"As of this time, we have found nothing that substantiates those reports," said Navy Capt. Curt Jones, commanding officer of Naval Base San Diego.
"I would characterize it as someone who thought they heard something," he added.
Jones said authorities at the base have conducted a "number of clearing sweeps."
"There are no casualties at this time," he said, adding that people were being extracted from the center in a safe and orderly fashion.
He said employees at the center are trained regularly and taught to "run, hide and fight."
"It appears that all the appropriate actions had been taken" in response to the report, he said.
The center has a 272-bed hospital and ambulatory complex, 11 primary care clinics and 10 dental clinics caring for active duty and family members and two centers for recovering wounded troops, according to its website.
More than 6,500 military and civilian professionals work at the center.
NBC News reported that a SWAT unit, California Highway Patrol officers and local police units have responded to the scene.
"Every week we have some kind of training" for an active shooter situation, an employee at the hospital's childcare center told NBC, adding that there are likely about 200 children at the center.
Naval Base San Diego posted on its Facebook page that all children at the medical center are "currently secured."
"All children are accounted for and safe. The entire complex is secured, there is no access to the facilities at this time," it said.
Earlier, the page instructed those inside to "Barricade the door and dial 9 1 1."
"Silence phones and other devices. Stay away from doors and windows. Remain calm and quiet. We are monitoring this page and will continue to provide updates," it had posted immediately after news of the shooting.
Naval Base Point Loma was also locked down, local outlets reported. Several nearby middle school and high schools were also on lockdown.
Jesse Byrnes contributed to this breaking news report, which was last updated as 1:42 p.m.