Emergency officials in Minnesota reduced the death toll from last night's bridge collapse to four this morning, but said as many as 30 people remained missing as divers continued to search among the shells of 50 cars that fell into the Mississippi River during yesterday's evening rush-hour.

Police Lt. Amelia Huffman said that last night's death toll - which was reported by local newspapers to have reached nine - was based on the best estimates of rescue officials. "This morning, the medical examiner’s office only has four sets of remains," she said.

But the chief of police in Minneapolis, Tim Dolan, said that divers were investigating the remains of "numerous" other vehicles in the water and that "people that may be in them". He said between 20 and 30 people were still missing.

Earlier, the Governor of Minnesota, Tim Pawlenty, told NBC television: "There’s no question that the fatality number will go up... 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."

Every ambulance in Minneapolis responded to the disaster, in which dozens of cars slid and fell 60 feet as the eight-lane Interstate 35W bridge buckled and collapsed just after 6pm local time (12.00BST).

Sixty people were taken to hospital, of whom six were later said to be critically injured. Among those hospitalised, but not seriously hurt, were ten people from a schoolbus carrying children from a summer camp that just avoided slipping into the wreckage. Photographs showed an articulated lorry burst into flames and cars crushed by rubble.

Overnight, police called off the search for survivors as 20 families gathered at an information center for news of the missing. "I've never wanted to see my brother so much in my life," said Kristi Foster of her brother, Kirk, whom she had been unable to contact since last night.

Police said the were treating the ruins of the bridge and its twisted steel trusses as a crime scene until the cause of the collapse had been determined. Recent structural inspections passed the 40-year-old structure as safe, but said that it was "structurally deficient" and would need to be replaced by 2020.

Governor Pawlenty confirmed that the US Department of Transportation's National Bridge Inventory gave the bridge's superstructure a rating of 4 out of 9, in which a zero means "failed".

He said that inspections in 2005 and 2006 had found no fundamental flaws and that work being carried out on the bridge at the time of the collapse had been superficial repairs to its deck, guard rails and lights.

Today one of the 18 contractors carrying "overlay work" on the structure recounted the moment of collapse. "I heard a low rumble and saw two shoots of concrete dust shoot out each side of the bridge," Xavier Sose told CNN. "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."

Last night, Progressive Contractors Inc., which was carrying out the work said 18 workers were on the bridge at the time of the disaster and that one of them was missing.

Federal safety officials are now on their way to Minneapolis to help investigate how the bridge failed. Known as "Bridge 9340", the structure was completed in 1967, the same year as the collapse of the Silver Bridge, in Ohio, that claimed 46 lives and prompted widespread concern over the safety of American highway bridges.

In 2003, the American Society of Civil Engineers has warned of corroding bridges and other US infrastructure, saying in a 2003 report that 27 percent of US bridges were structurally deficient or "functionally obsolete" due to outdated designs.

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