Brent -- I didn't hear ball bearings, but I did hear glass -- also, the placement lends itself to shrapnel-by-proximity.
Obviously a terrorist attack -- the venue was not only chosen for its large collection of people in one place, but the mood-darkening aspect of such an attack at such an event, and the fact that it would almost definitely be documented. The only question remains is who is responsible, domestic/foreign, individual/organization, etc. But very carefully thought out whichever way it goes.