There are key points for the NSA to intercept communications. The theory is that the AT&T and the NSA intercept points are connected somehow. If this is true, the "bombing" was about poking out an NSA "eye" to hide something.

Also, note that the communications building that was hit was battle hardened, so a typical bomb would not hurt it. It correlates with the theory that the RV was just cover, and the real explosion was from a bunker buster missile.