Horror details contained in a “manifesto” left behind by the suspects of a deadly shooting outside a US mosque have been revealed by the FBI, who say the two teenage gunman appear to have been radicalised online. Three adults, including a “heroic” security guard, were killed in the attack on the Islamic Centre of San Diego, which houses a school, on Monday US time.The two teenage suspects, Cain Clark, 17, and Caleb Velasquez, 18, were later found dead inside a BMW a short distance from the centre.In a press conference on Tuesday, the FBI revealed officials recovered a “manifesto” which covered “a wide aspect of races and religions”. “We are dedicating every resource the FBI has to conduct a thorough analysis of that manifesto to try to learn what led to this, but I think also more importantly, how can we stop future attacks,” Mark Remily, the special agent in charge of the FBI’s San Diego office, told reporters, “They didn’t discriminate on who they hated.”Mr Remily said it was too soon to say if the Islamic Centre was the “specific target” of the attack.“We are assessing and analysing all the evidence,” he said. “What I can say is they definitely had a broad hatred towards a lot of folks.”Law enforcement also seized more than 30 guns, including pistols, rifles and shotguns, as well as a crossbow, tactical gear, ammunition and electronics when executing three search warrants at locations associated with the suspects.Gunmen met, radicalised onlineThe two alleged shooters appear to have met online and discovered they both lived in the San Diego area, Mr Remily said.“They became acquaintances or associates and had in-person contact,” he said.He added the pair appeared to have been radicalised online, however, authorities are still investigating how the radicalisation occurred.“The victims who lost their lives yesterday were there to help others be part of a community that came together in peace,” Mr Remily said. “Instead, they were confronted by teenagers who appeared to have been radicalised online to believe that they didn’t belong because of how they looked or where they worshipped. They couldn’t be more wrong.”Clark attended Madison High School, where he was a standout wrestler. His grandfather David Clark, 78, said the family was devastated. “We’re very sorry for what happened. We know as much as you do. It’s a shock,” he said.Victims praised for saving lives Police credited the three victims – Mansour Kaziha, Nader Awad and security guard Amin Abdullah – for distracting the suspects and saving countless lives during the attack. “All three of our victims did not die in vain,” said San Diego Police chief Scott Wahl.“Without distracting the attention, without delaying the actions of these two individuals … without question, there would have been many more fatalities yesterday.”In particular, police praised Mr Abdullah, calling his actions “heroic”.The father of eight exchanged gunfire with the two shooters and was killed outside the centre. Before he died, he issued a lockdown protocol, warning teachers to lock the classroom doors of the mosque’s school.“The sound of the gunfire, all of the delay in that gunbattle, allowed everybody to get out of the main common areas of the mosque and into areas of hiding,” Mr Wahl said. “So, when they began to go through the areas of the mosque, they were not running into more victims.”He said security footage shows the gunmen going “room to room”, but they couldn’t find anybody. “If it was not for (Mr Abdullah) … the carnage would be much worse,” said Taha Hassane, imam at the Islamic Centre.“He’s the one who stopped them, who slowed them down. If he didn’t do what he did, and sacrifice his life, the two suspects would have (had) easy access to every single classroom.“We’re so proud of him. I envy him when I see messages about him literally from all over the world talking about his heroism.”Mr Abdullah’s daughter told the press he was a “loving father, husband, son, brother, and uncle”.“To me, my dad was a role model,” Hawaa Abdullah said. “He was a best friend. He was the best, absolute best dad in the world.“My dad was the number one advocate for safety and keeping our community safe. He stood against any form of hate. He took his job seriously to protect everyone here.”The other two victims – Mansour Kaziha and Nader Awad – drew the attention of both the shooters away from the mosque and into the parking lot, where they were both killed.“Both suspects were able to successfully corner them and kill both of them,” Mr Wahl said.“While both suspects were outside, the massive response that was seconds away undoubtedly pushed both of the suspects to run back to the vehicle and flee the parking lot.”Call from concerned mother before shooting Just hours before the shooting, police said they received a call at about 9.42am from a concerned mother reporting a runaway juvenile, who has been identified as one of the suspected shooters.“She believed her son was suicidal and she began to share information that several of her weapons were missing – her vehicle was missing – in addition to her son,” Mr Wahl said on Monday. She added that her son was accompanied by another person and that both were dressed in camouflage.Later that morning, at about 11.43am, officers responded to reports of an active shooter at the centre.Police arrived within four minutes of the first emergency calls, they said.When officers arrived, they found three people dead outside the centre, prompting them to engage in an “active shooter response” inside the complex.“They did have to breach doors to get into all of the different rooms,” Mr Wahl said, adding it was “extremely chaotic”.“There was somewhere between 50 to 100 police officers inside of that facility.”Active gunfire rang out a few blocks away, where a landscaper was fired at but was not injured, said Mr Wahl.