Perhaps a Muslim or two on the janitorial staff?
I hope someone is going over the list of employess and checking names with times of shifts and who checked in and out. Of course, if someone had nefarious deeds in mind he (or she?) may not have time clocked accurately.
But the nature of the explosions - from what has been posted - sounds as though it could as easily have been done a-purpose as not. If in order to create a huge explosion, all that's needed to is get some gas to leak, and then arranage a little timed device, or even wait for nature to take its course, who would ever know?