SAN DIEGO — Like schools across the U.S., the Islamic Center of San Diego had sought to prepare its students for the possibility that a gunman could breach its walls.

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When two armed teenagers stormed the mosque Monday — and those “active shooter” drills were put to the test — the young students in one classroom did exactly as they had practiced and “went straight to business,” their teacher’s assistant told NBC News.

“If they didn’t, then this could have been a different outcome,” said the assistant, Imani, who teaches second and third graders and asked to go by her first name.

Authorities have said the three people gunned down at the mosque, described on its website as San Diego County’s largest, were all outside. No one who was inside the center at the time — including dozens of students attending school — was injured, San Diego Police Chief Scott Wahl has said.

One of the victims, security guard Amin Abdullah, fired at the teens when they sought to enter the mosque. He then used his radio to trigger a lockdown protocol, Wahl told reporters.

An imam at the mosque, Taha Hassane, said the center practices the lockdown drills multiple times every school year. Abdullah, he said, had been with the mosque for several years and knew about the system, which notifies teachers in their classrooms about active shooters.

The gunmen returned fire, as did Abdullah, who was killed in the gunfight, Wahl said.

Wahl said Abdullah “delayed, distracted and ultimately deterred” the shooters from gaining access to the areas of the mosque where there were as many as 140 kids.

As soon as Imani heard the gunfire, she recalled, she looked at a colleague, and they immediately knew what was happening.

“We told the kids this is not a shooting drill,” she said. “There is an active shooter, and let’s go.”

The students did as they had done in practice drills, she said, filing into a corner of the classroom, staying low to avoid windows and making sure the doors were shut and locked.

The students remained silent, even as they could hear the shooters banging on doors and trying to open them, she said.

“We are just so proud of them and their bravery,” Imani said, adding: “They held it together.”

Some of the security video from inside the mosque showed the gunmen moving from room to room, Wahl said, but those areas were empty. The shooters ultimately found two men — Mansour Kaziha and Nader Awad — in the parking lot outside and fatally shot them, Wahl said.

Each of those victims is “worth more than 1,000 men,” Imani said. “They are the reason all 140 of us made it out alive.”

Authorities said the gunmen, identified as Caleb Vazquez, 18, and Cain Clark, 17, are believed to have taken their own lives after they fled from the mosque.

Authorities investigating their motives are trying to authenticate a 75-page document that they may have written and posted online, law enforcement officials have said.

The material espouses anti-Islamic, antisemitic and anti-LGBTQ views, and it refers to accelerationism, a white supremacist ideology that promotes violence to speed the formation of a white ethnostate.

“These subjects did not discriminate on who they hated,” an FBI official, Mark Remily, told reporters Tuesday.

Morgan Chesky reported from San Diego and Tim Stelloh from Alameda, California.