A security guard at the mosque, Amin Abdullah, exchanged gunfire with the attackers and initiated a lockdown, helping protect around 140 children who were inside the building.
Abdullah and two worshippers – Mansour Kaziha and Nader Awad, who drew the attentions of the shooters in a parking lot – were fatally shot.
San Diego Police Chief Scott Wahl previously described Abdullah’s actions as “heroic”, saying they had “undoubtedly” saved lives.
According to investigators, the shooters recorded the attack using cameras mounted on GoPro-style helmets. O’Neill said the pair had also planned further attacks “at a local Jewish temple and finally at a predominantly African American high school”.
Authorities said the teenagers had an arsenal of around 30 guns and a crossbow, which investigators say were taken from one of the shooter’s parents.
The attackers also left behind writings containing racist and antisemitic views, as well as references to white supremacist ideologies.
The document, which was reviewed by the BBC’s US news partner CBS News, pointed to a gunman who murdered 51 people at two mosques in Christchurch, New Zealand, in 2019, as one of their models.
