The US congress is working on a $250m federal aid package for the disaster which happened on the Interstate 35W bridge during rush hour on Wednesday.

 
The official toll stands at four, with 79 injuries.

 

Toll to rise

 

Rescuers are still searching for bodies after spotting dozens of submerged vehicles which plunged about 20 metres into the Mississippi river following the collapse.

 

"We thought we had done all we could. Obviously something went terribly wrong"

Dan Dorgan, Minnesota bridge engineer

About 15 divers and a dozen boats were in the water but strong currents and low visibility was slowing down the search.
 
Tim Dolan, the Minneapolis police chief, said the number of casualties was expected to rise as many vehicles were still submerged under big pieces of concrete.
 
"We do know we have some people in those vehicles. We know we do have more casualties at the scene," he said.
 
Tim Pawlenty, Minnesota's governor, said: "There's no question the fatality number will go up."
 
Corrosion and fatigue
 
The 1990 inspection cited significant corrosion in the bearings of the collapsed span, a finding which required portions to be repaired or replaced, and inspections every two years.
 
Subsequently, fatigue cracks and corrosion in the steel around the bridge's joint were repaired but a federal inspection in 2005 still rated the bridge structurally deficient.
 
States have been asked to inspect more than
70,000 bridges across the US [GALLO/GETTY]
Dan Dorgan, a Minnesota bridge engineer, said: "We thought we had done all we could. Obviously something went terribly wrong."
 
The collapsed bridge last underwent a full inspection on June 15, 2006 and was not scheduled for replacement until 2020.
 
George Bush, the US president, said the federal government would help rebuild the bridge, but made it clear the state government was responsible for fixing the structural deficiencies on the eight-lane bridge.
 
"We in the federal government must respond, and respond robustly, to help the people there not only recover, but to make sure that lifeline of activity gets rebuilt as quickly as possible," he said.
 
He is scheduled to visit the scene of the disaster on Saturday.
 
High cost
 
Engineers estimate that repairing the more than 70,000 bridges with structural deficiencies would take years and cost more than $188bn.
 
The American Society of Civil Engineers said that works out to at least $9.4bn annually over 20 years.
 
In a separate cost estimate prepared five years ago, the Federal Highway Administration said it would take at least $55bn to address the backlog of required bridge repairs.
 
Gregory Cohen, president of the American Highway Users Alliance, an advocacy group representing motorists, said: "We're not doing what the engineers are saying we need to be doing."
 
He added that engineers have estimated $75bn a year just to keep highways and bridges from further deterioration, but only about $60bn a year is being provided.