I used to drive over that bridge quite often when I lived in the Twin Cities. If you ever have the opportunity to be in downtown Minneapolis, walk to the Riverfront and look at each bridge that crosses the Mississippi River. You'll note every bridge is a little different from the others. The Hennepin Ave. bridge is a suspension bridge. The University Avenue bridge I remember as a stone arch bridge. The I-35W bridge that collapsed was a steel girder bridge in an arch array.
I wouldn't be that surprised if MnDOT, the state's highway department, comes out with some kind of report that the design was faulty. If not that, I will imagine the issue is that the materials used couldn't withstand the extreme temperatures over time. In Minnesota you can have days on end with temps as low as 15-30 below zero, then you can hit 90-95 in the summer.
I saw a written report out of the Star-Tribune tonight that quoted some one who escaped the collapse at the last moment. That person said she saw a MnDOT worker jack hammering the road surface. Don't laugh, but maybe one wrong "jack" and a chain reaction may have started which led to this tragedy.
Watch this story over the next few days.