If Ammonia Nitrate is being stored and the side of storage unit has been penetrated by forklift or vehicle then this would lead to pellets lying outside . Fireworks trigger external Ammonia Nitrate pellets which in turn trigger storage facility , game over . I dont know and am only guessing but I have seen some very loose practices in the past . I imagine an investigation will reveal . I do think the US could have satellite footage maybe ??
