A couple of disgruntled, radcal muslims killed some of their coworkers, just like it has happened before in this country, and will happen again, just like a white radical christian asshole killed innocent people in Colorado.
Terrorism is in the underground in America. The common denominators are radical jihadism, muslim or christian, and accessibility to weaponry, especially heavy weaponry.
And these are the unnassailable facts, and we better deal with them.