Most bridge collapses occur from an obvious cause: an earthquake or a barge running into bridge supports. On rare occasions, however, bridges have collapsed for less obvious reasons. In 1983 in Greenwich, Conn., the Mianus River Bridge collapsed, killing three people. A federal investigation blamed excessive accumulation of corrosion on a hangar pin, a key part of the bridge.

In the case of the Minneapolis bridge collapse, the National Transportation Safety Board will lead the investigation. Its investigators were also on the scene to begin to piece together what had caused the collapse.

"It is much too early in this investigation to know what happened," NTSB Chairman Mark Rosenker said Thursday. The first step is to reassemble the pieces of the bridge like a jigsaw puzzle to figure out what triggered the collapse, he said.

State inspection officials had inspected the bridge twice since the federal government rated the bridge "structurally deficient" but concluded the bridge was safe. State officials were in the process of completing a third inspection, interrupted because of construction on the bridge, when the bridge collapsed Wednesday afternoon at the height of rush hour.

As many as 30 people were missing as of press time Thursday.

Concerns about the bridge go back at least six years.

A 2001 report by the University of Minnesota, Department of Civil Engineering, stated, "Concern about fatigue cracking in the deck is heightened by a lack of redundancy in the main truss system. Only two planes of the main trusses support the eight lanes of traffic. The truss is determinate and the joints are theoretically pinned. Therefore, if one member were severed by a fatigue crack, that plane of the main truss would, theoretically, collapse."

This was a steel, arch-truss style bridge with a concrete deck that should have lasted at least 60 years, says P.K. Basu, a civil engineer at Vanderbilt University and an expert on bridge design and failure. Corrosion of rivet connections is a suspect, as are possible cracks around such joints. Some signs of structural failure due to corrosion are subtle, he says, and may only be discernible by experts. Increased weight of trucks in recent years could be another factor.

The bridge was part of I-35, a major transportation link for Minneapolis, and one of the most heavily traveled urban highways in the country. It was also the first of its size in the United States to be equipped with an anti-icing system that sprayed a de-icing element on the bridge deck.