After reading the report, I have absolutely no doubt that terrorism was NOT involved. The mystery is: “how did it stay up that long”, not “why did it fall”.
As far as “what kind of knowledge and equipment would be needed to take out a bridge like this”: A fair amount of both. Any structural engineer could pinpoint weak points in any bridge. This one, being fracture-critical and non-redundant would have been particularly easy — just about anywhere you look. Redundant and NON-fracture-critical bridges would be a lot harder.
Even on this one, the amount of equipment needed would have been quite a bit and anyone doing something would have to have been out where anyone could see them (the best place would have been the lower chords between the main bridge piers). Of course, explosives don’t take up much space, but there is no sign of explosives being used here. Others have suggested a cutting torch. Those box beams that made up the truss were large enough for a person to get inside. You would have to cut through at least half the entire beam to guarantee a failure. That would take some time.
How about either Thermite (dump it onto a good spot, light a fuze, and leave -- but the melting would be obvious), or putting some corrosive in a weak spot to greatly accelerate the impending failure?