BREAKING : Video footage released by USA Today shows teens Cain Clark, 17, and Caleb Velasquez, 18, firing from a vehicle outside the Islamic Centre of San Diego during the May 18 attack.The pair live-streamed the shooting, killing 3 people including a security guard and two school staff members before dying from self-inflicted gunshot wounds.Investigators say neo-Nazi symbols and a manifesto linked to Christchurch attacker Brenton Tarrant were found.