gotta be someone who works there. i was slightly amused at all the alphabet agencies they listed as "investigating". Not that ive ever made one of these but i think i saw someone do it when i was like 12. They only take a few minutes at most before they go off so you'd have to be fairly close nearby. I know it could hurt the person holding it but it's unlikely to hurt anyone else outside of a few feet besides scaring the bejesus out of someone.
either inside job for a prank (which will and should result in jail time) or terrorists testing the waters... several were in non public areas werent they?