Groton — Less than two weeks ago, reports circulated of an active shooter at the Navy Medical Center in San Diego. 

Officials found no evidence of a shooting, but the threat is on the minds of personnel at all Navy installations, including the Naval Submarine Base.

The base, in the midst of a regularly scheduled Navy-wide security exercise, carried out an active shooter drill Thursday in the main hall of the Morton Hall Gymnasium.

Just after 8:30 a.m., the sound of rapid gunfire rang out over the gym's public announcement system.

A group of young sailors who were playing basketball scurried to hide behind whatever they could find — bleachers, the base of a basketball hoop.

From behind a row of elliptical machines, the shooter — dressed in a black sweatshirt and dark gray sweatpants — fired shots from an Airsoft gun meant to resemble a semiautomatic or automatic weapon.

The gym proved to be a good training ground for the drill because the acoustics there caused an echo effect, which made it difficult for the responding security force team to determine where the gunshots were coming from.

Two security personnel, armed and wearing bulletproof vests, ran into the room firing at the shooter until she fell to the ground, dropping her gun on the way down.

One of the men kicked the gun away from the shooter and proceeded to put her in handcuffs.

A handful of bodies lay dispersed around workout equipment. They each had varying degrees of injuries, and one of them was dead.

The backdrop for the drill was that the shooter, described as a loner sailor, had been communicating with "bad guys" over the Internet and planned to carry out an attack.

"There's no way that I know of all the people who work on this base, who's disgruntled," Capt. Paul Whitescarver, commanding officer of the base, said. "Can I defend against that? (It's) very difficult.

"Can I defend against a person who shows up at a gate with a gun? Yeah, because I can shut my gates," Whitescarver said. "But a person who's disgruntled inside the fence line, that's tough."

In situations like these, Navy personnel are advised to either run, hide or fight, according to Department of Defense and Department of Homeland Security policy.

In the Navy Medical Center incident, a post on the center's Facebook page alerting people of the reported shooting, also advised all occupants to run, hide or fight.

Whitescarver said the base would also use Facebook to get the word out if a similar situation happened here.

In addition to run-hide-fight, the active shooter training that Navy personnel receive encourages individuals to "notice coworkers and recognize possible unusual behavior or out of the norm actions/mood swings, etc.," base spokesman Chris Zendan said by email.

Outside the gymnasium, base officials activated the Emergency Operations Center, or EOC, in a separate building.

Once it was activated, Cmdr. Kurt Stronach, executive officer of the base, immediately set four goals: eliminate the threat, isolate the scene and make it safe and secure, respond to the injured, and isolate the gym so that the rest of the base could continue its mission.

EOC staff comprises base leadership, logistics, fire and security personnel, a representative from Fleet and Family Support Center, and public affairs officials, who all have different roles to play in the event of an emergency.

From the time the shooting occurred, it took about 18 minutes for the EOC to be fully staffed. If someone were unable to make it for security reasons, somebody else in the EOC would take over those responsibilities.

The staff debriefed after the drill. They discussed the uniqueness of the scenario given the close proximity of the gym to the building that houses the EOC.

They talked about activating the virtual EOC under these circumstances, or using the alternative EOC, which is housed on a separate building on base.

"Two years ago, I would've told you this is a far cry that this would ever happen," Stronach said during the debriefing. "Unfortunately today, I stand here telling you we're almost at even odds that somebody is going to get disgruntled and this is going to happen. And so we got to be ready."

After the 2013 shooting at the Washington Navy Yard, the Navy required that all of its personnel in the U.S. complete active shooter training.

And after last summer's shooting at a military recruiting station and a second military site in Chattanooga, Tenn., the Navy required that all of its personnel stationed around the world complete the training.

Locally, the base has held active shooter drills for at least 10 years, according to Zendan.

Whitescarver doesn't have legal jurisdiction over its child care center and housing communities, where nearly 1,800 people live, both of which are outside the base.

Of the child care center, he said, "That's one of my big concerns. That's a soft target to me. I'm still struggling with how all that's going to play out."

The Town of Groton has jurisdiction over those areas, so if there were a security issue, Whitescarver would rely on the town to respond.

The base's fire department would also show up to "give us an incident command post," and base officials would assist town officials.

"You know, these events occur, they're random," Whitescarver said. "You never know where they're going to happen so the only way that you can make sure that you respond correctly is to practice it."

j.bergman@theday.com

Twitter: @JuliaSBergman