Rescuers searched on Thursday for 20 people missing in a U.S. bridge collapse that sent vehicles tumbling several stories into the Mississippi River in an avalanche of concrete and steel that killed at least four, authorities said.

The 40-year-old steel truss bridge in Minneapolis buckled without warning during Wednesday's evening rush hour, and witnesses caught in the catastrophe recalled people screaming, We're going into the water.

Authorities put the death toll at seven on Wednesday evening but revised the figure in the morning, CNN reported.

There's no question that the fatality number will go up, Minnesota Gov. Tim Pawlenty told NBC's Today show.

And we know there are a number of cars in the water that we haven't been able to get to and they've been there submerged since last evening, he said.

Local officials said 20 people were missing on Thursday.

Several workers that had been doing resurfacing work that had closed some of the span's eight lanes were among those plunged into the river when the bridge buckled and fell some five stories and onto adjacent embankments.

I heard a low rumble and saw two shoots of concrete dust shoot out each side of the bridge, Xavier Sose told CNN the next morning. Next thing you know I hear another rumble, complete silence.

And then I just see the bridge disappear in front of me. I was dumbfounded.

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Pawlenty said a U.S. Department of Transportation report in 2005 gave the bridge poor marks, designating it structurally deficient and likely in need of replacement.

On a scale of zero to nine, with zero meaning failed, the bridge's superstructure was rated at four, the Minneapolis Star-Tribune reported.

The governor said the bridge had passed inspections in 2005 and 2006.

He said the U.S. Army Corps of Engineers would use barges to remove debris from the now-closed section of the river, a vital commercial artery in the country's midsection.

Federal safety officials were on their way to the Midwestern city to help investigate. Investigators are looking at whether bridge construction played any role.

I unfortunately can't say how it happened right now, Mayor R.T. Rybak said in a television interview Thursday.

Some of the roughly 60 injured were pulled from half-submerged vehicles, some swimming to safety. Witnesses estimated there were 50 to 60 vehicles on the bridge at the time.

A school bus carrying mostly children from a swimming outing landed on its tires, and the 59 children and adults on board scrambled out the back exit, bloodied and bruised.

There was smoke and noise. Everybody was screaming, said one girl who had been on the bus.

Jay Danz, who had been driving on a roadway below the bridge just before it collapsed, told the Star-Tribune: I heard it creaking and making noises it shouldn't make. And then the bridge just started to fall apart.

Between 100,000 and 200,000 vehicles normally pass over the bridge on a given day.

(Additional reporting by David Morgan and Doina Chiacu in Washington)

Safety