OBTW Harry Reid's office is in the building
http://www.lvrj.com/news/Two-FBI-agents-shot-in-downtown-Las-Vegas-80624367.html
That would explain it. Reid sends in a bunch of masked men which has been reported to be anywhere from 1 - 4 men with guns with anywhere from a couple rounds to 50 rounds being fired. This helps his anti-gun plan and also paints him as a possible target to get the sympathy vote.
That sneaky bast-rd.