"Because they came to recognise the ponies moving in and out of there, and got lazy. What we do know is that a stallion they didn't recognise entered the offices this morning, with the right passes, and since he didn't leave, we're assuming that the scant remains of the pony we found at the bomb site was in fact the bomber. Whether he intended to die in his own attack we don't know, but this does mean we have no way of finding out who he is, or why he did it."