I believe you failed to sense my point. My question for you is this: Would you be more or less alert after the initial explosion?
You're assuming there's anybody to BE alert. We don't even know if anyone on the island knew about the Oslo bombing.
I just don't get the astonishment that the island didn't have security. Lets say a bomb goes off in downtown DC; 90 minutes later, is some summer camp for kids in the Blue Ridge Mountains of Virginia going to be on some sort of security alert with armed guards at all the entrances?