I've heard - through unofficial sources of my own, which may or may not be accurate - that it was a "fuel-based" explosive. That could mean a few things, but it was most likely another fertilizer bomb like Nichols and McVeigh used in Oklahoma City. I still think it's unlikely one mand could build a bomb like that alone, but I suppose if he was working on it for a year and a half, it's possible.

He must have had some sort of beef with AT&T. After all, everyone else does. But I think his main motive was to go out with a bang. Clearly, he did.

Onward and upward,
airforce