Terrorism was ruled out, with authorities stating that it appears to be a structural failure.
It is believed that there were approximately 50 vehicles on the bridge when the span over the Mississippi River collapsed. Eyewitness reports say that traffic was moving at approximately 5 to 10 miles per hour when the bridge fell.
UPDATE: Design News asks the question “Was it structural failure or metal fatigue?”
There are also a number of related stories that can be followed from the link above.