[PapaBear3625]

While it’s true that anyone can rent a torch, cutting steel isn’t a low profile operation, and you don’t just whisk through three quarter or inch thick plates like a knife through butter either.

How about if somebody used thermite? (nice video here )

[concretebob]

or powdered magnesium in a paste.
Wouldn't take long to fry a web or flange.
Sabotage or terrorism is way down the list of possible causes, given the real-time conditions.
There was enough going on on that bridge to create a cumlative effect.
Too much stuff in one spot, on a bridge that already had a less than 50% safety rating.