I am actually shocked they did not take out the AMEX building itself. That would have been a HUGE SCORE. As for any concerns there about account numbers, let me assure you unless they had security clearance through their loss prevention or AMEX Police team (which they have their own law enforcement there) a code comes up and flashes right away if they are looking into any famous person's account, or looking up a non-callers account. They then are charged with a crime and dismissed immediately. The size of this complex and the parking garage would have been something to blow up.
That would have had a major hit on people's financial worlds as well because if I am not mistaken it is their main HQ.
Actually AMEX has another large office in Phoenix that I used to deal with all time in my lsat job. Cyber attacking AMEX computer systems to steal money and completely destroy their account records would do much more damage than blowing up the buidling.