Speculation is futile. Only the law enforcement agencies involved have all pertinent data.
My heart goes out to all those killed, their friends and family. All of you in that area, take care.

An aside: I just received an invitation to the Nov. 11th commemoration ceremonies at the Vietnam veterans Memorial (The Wall) in DC. I wonder if they will continue plans for the ceremony out in open?
Carrying on so as not to give in to terrorism is laudable but there is such a thing as common sense!