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Toivonen said that below him 40-50 feet was a tabletop of concrete. He scrambled down to the scene and found a man who he said, "had a chunk of a beam on his arm and a piece of concrete on his head."

The cause of the collapse wasn't known in the hours afterward. It's too soon to know what happened, said Catherine E. Wolfgram French, a civil engineering professor at the University of Minnesota.

"Things can happen with temperature, and with construction, or a lot of other confounding factors," French said.

This was a 40-year-old truss bridge, and French did say that some early truss bridges don't have as many structural redundancies -- backups to carry the loads -- as is now considered desirable.

Another engineer, Michael Ramerth, a principal at MBJ Consulting Structural Engineers in Minneapolis, said in the search for answers "I would start at the foundations."

On a typical weekday, more than 100,000 cars use the bridge.

Berndt Toivonen, 51, of Minneapolis, was on his way home from a painting job when the bridge collapsed beneath his car.

"The bridge started to buckle," Toivonen said. "It went up and came down. I thought I was going to die."

Bumper-to-bumper traffic

What people in the area of the collapse experienced or saw at about 6:05 p.m. unfolded as motorists crawled bumper to bumper across I-35W toward the end of rush hour.

Those on the bridge felt buckling and swaying and heard a crunching.

Then came the unthinkable: The 40-year-old bridge collapsed, dumping vehicles into the water and onto land below. That was followed by scenes of frantic, bloodied motorists and rescuers who converged on the scene.

Many vehicles, including at least one semitrailer, were on fire. People were reported to be floundering in the river. Rescuers rushed to help people escape cars trapped in the V-shaped hollow where the bridge had caved in.

The school bus that fell was, returning from a day-camp swimming trip sponsored by a Waite House summer program.

"We collapsed," said Ryan Watkins, one of the children.

Crumpled wreckage lay on the east bank of the river, and a huge section of concrete roadway lay on the west bank. Down below in the river gorge, rescue workers scrambled to help people get out of the water.

Fire and black smoke rose from the wreckage.