Three people were shot and killed at the Islamic Center of San Diego on Monday by two teenagers who were subsequently discovered dead inside a nearby vehicle, according to The New York Times. The shooting is being investigated as a hate crime.

The victims were identified as Amin Abdullah, a security guard; Mansour Kaziha, the mosque’s store manager and caretaker; and Nader Awad, a worshiper who had come to help, according to The Times. Before he was killed, Abdullah triggered a lockdown alert that, combined with the sound of gunfire, enabled roughly 140 children present in the building to shelter in place, Police Chief Scott Wahl said, according to ABC News. Having moved through the mosque without encountering anyone, the suspects caught sight of Kaziha and Awad from a window, then rushed out a nearby door and cornered the two men in the parking lot, where they were killed, according to ABC News.

The suspects were identified as Cain Clark, 17, and Caleb Vazquez, 18. According to The Times, the two had first connected on the internet and, upon realizing their proximity to one another in the San Diego area, eventually met face to face. Footage from what appears to have been a livestream of the assault captured the pair outfitted in camouflage and tactical gear adorned with white supremacist insignia. The video showed Clark fatally shooting Vazquez before taking his own life, according to ABC News.

A 75-page document recovered from the suspects’ vehicle, which investigators characterized as a manifesto, laid out a white nationalist ideology and contained extensive screeds targeting a range of racial and religious communities, according to CBS News. The FBI’s special agent in charge of the San Diego field office, Mark Remily, said the suspects “did not discriminate in who they hated,” according to CBS News. Searches of properties connected to the two suspects turned up an arsenal that included upward of 30 firearms and a crossbow, according to ABC News.

That morning, Clark’s mother had contacted police out of concern that her son had left home in camouflage, taking her car and several weapons with him, according to CBS News. A search for the vehicle was still underway when, roughly two hours after that call, gunfire broke out at the mosque.

The Islamic Center of San Diego is the largest mosque in San Diego County. Law enforcement agencies across California and in other parts of the country announced stepped-up presence at mosques and other religious sites in the wake of the attack, according to The New York Times. Muslim advocacy groups said the killings reflected a disturbing buildup of anti-Islamic hostility, citing a surge in online hate speech and increasingly inflammatory rhetoric from some politicians in the months prior, according to The Times.