The two teenage assailants responsible for a mass shooting at the Islamic Center of San Diego, California, rushed toward the mosque “fully armored” with handguns and rifles, authorities said.

A security guard shot and struck one of the shooters, according to members of the mosque, but the attacker continued charging. The guard, Amin Abdullah, alerted administrators of the school at the Islamic Center, telling them to go into lockdown, before he was shot and killed. “If it was not for him … the carnage would be much worse,” said the imam, Taha Hassane. “He sacrificed his life.”

Two other victims, identified by the Islamic Center as Mansour Kaziha and Nadir Awad, were on the phone calling the police when they were killed. A witness shared details of what he saw transpire in an Instagram post from the Islamic Center.

Police rushed to the scene and ultimately found the suspected shooters near the masjid, dead from self-inflicted gunshot wounds.

The Islamic Center called the three victims “three heroes martyred” and has started a fundraiser for their families and for others affected.

The shooters had met and been radicalized online, according to the FBI. At a news conference on Tuesday, Mark Remily, the special agent in charge of the FBI San Diego office, said investigators uncovered a “manifesto” that expressed a “broad hatred” toward various races and religions.

Investigators found 30 firearms and a crossbow after searching two residences in connection with the investigation. Remily said agents also seized tactical gear, ammunition and electronics.

“We also identified writings and various ideologies outlining religious and racial beliefs of how the world they envisioned should look,” he said. “These subjects did not discriminate on who they hated.”

He said it was still too early in the investigation to say whether the Islamic Center was the sole or specific target of the shooters.

Authorities have not yet publicly shared the names of the suspects, but they have been identified in news reports as Cain Clark, 17, and Caleb Vazquez, 18.

Todd Blanche, the acting US attorney general, said his office, the FBI and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives were working “with local law enforcement partners to fully investigate the circumstances around this horrific and senseless attack on a religious institution”.

On Monday, the mother of one of the shooters alerted police soon before the shooting occurred that her son, a 17-year-old high school student, was suicidal and potentially armed. Officers were looking for the 17-year-old and his 18-year-old friend in a neighborhood near the mosque as they opened fire.

Remily said the firearms the shooters used were registered to one of their parents. “Exactly how they … were able to obtain them is still under investigation,” he said. The FBI has opened a tip line and is still seeking more information about the shooters and their motives.

One of them had been enrolled in a virtual learning academy at the San Diego unified school district, according to a spokesperson for the district, and had been due to graduate high school this year.

During the shooting, five public schools near the Islamic Center were placed on lockdown. Hassane, the imam and director of the Islamic Center, said the center practiced active-shooter lockdown drills several times a year, which he said undoubtedly protected people.

“It worked, you know,” he said. “The lives of the kids were saved. The lives of everyone in the school were saved, and we’re so grateful for that.”

Outside the mosque on Monday, emotions were raw and unfiltered as the community came to terms with what had happened.

Maya, a seventh-grader, whose mother gave her permission to speak to the media, said she knew the three victims from family gatherings and from Ramadan, when she would see them every day. “He sacrificed his life to save everyone else’s,” she said about a security guard who was killed.

Jesus, a community member who preferred to go by only that name, said he would be back at the mosque as soon as it opened again.

“The Islamophobia going on in this country – that’s not going to stop me from praying five times a day, it’s not going to stop me from standing with my community. It’s not going to stop me from advocating for our people,” he said. “If anything, it’s making me want to do it more.”

The Associated Press contributed to this report.