I just watched a blurb about the MNDoT Bridge List, and they showed some pictures of the structure of this one from (I believe) the 2005 report. [ It runs as "Bridge History" under KARE-11 OnLive ^ ] They showed missing bolts, severe corrosion, and misaligned members in the steelwork support structures.
I'm sure there are probably more than a single cause to the failure, just as there is a multiple cause for the dearth of maintenance to the infrastructures -- self-aggrandizing politicians, money-grubbing lobbyists, and an uninterested public.
You’re welcome.