As I understand it:
The bomb was in a car and on a timer. By the time it went off, the bomber was well on his way to the camp ground where he presented himself as a police officer instructing all in security measures re: the Prime Minister's (IS that his title) visit.
Perfect sense even if they had heard of the bombing. All gathered round and he opened fire. Panic. Staff as well as children were slaughtered.
Well planned, mg.
Like it or not, no one was at fault.
Sometimes, shit happens.