Dozens of vehicles fell into the river or were crushed as parts of the eight-lane bridge, which rises 64 feet (20 metres) over the water, were sheared off during Wednesday evening rush hour in Minneapolis.
"The bridge started falling, cars were flying everywhere and I saw the water coming up," said Catherine Yankelevich, who was driving across the bridge when it fell.
Her car ended up in the water but she managed to roll down her window and swim to safety.
Authorities confirmed nine dead and said 20 people were missing, adding that they expected the death toll to rise, the Minneapolis Star-Tribune newspaper reported Thursday.
After four hours of frantic rescue efforts before night fall, the head of the fire department said the focus on Thursday would shift from search to recovery.
Jim Clack said more than 60 people were taken to hospital and it was unlikely that any more survivors would be found.
"We've ceased river operations for the evening. It's too dark. There's too much debris in the river to continue," he said Wednesday evening.
"We have moved from a rescue mode... to a recovery mode."
In Washington, Department of Homeland Security officials said there was no sign that the bridge's collapse was the result of terrorism.
"This is a catastrophe of historic proportions for Minnesota," said Minnesota Governor Tim Pawlenty.
The governor said the 40-year-old steel arch bridge was last inspected in 2006 and no structural problems were found.
There was, however, work taking place on the bridge "relating to concrete repair and rehabilitation and replacement, guard rail replacement, righting replacement and work on the joints," he said.
A 2001 report by the state transport department concluded that the bridge "should not have any problems with fatigue cracking in the foreseeable future" and recommended not to replace the bridge "prematurely."
But it also pointed to problems with corrosion in the bridge's steel beams, "poor welding", and said it was designed under 1961 regulations that have since been rewritten with stricter rules.
The American Society of Civil Engineers has warned of corroding bridges and other US infrastructure, saying in a 2003 report that 27 per cent of US bridges were structurally deficient or "functionally obsolete" due to outdated designs.
It was a disastrous scene at the bridge site as injured people crouched among the smoke on crumpled concrete, with steel girders submerged in the brown river.
Rescue workers tied with yellow rope waded through the water and used boats to reach people stranded in the middle of the river.
A school bus loaded with 60 children from a church group could be seen on the bridge but did not fall into the river as it collapsed. The children were all rescued, although ten of them were taken to hospital, the Red Cross said.
As a result of the repair work taking place, only one lane was open in either direction when the bridge crumbled shortly after 6:00 pm (2300 GMT), right in the middle of the local rush hour.
Ramon Houge was driving on the bridge when he heard a rumbling noise, saw the ground collapse in front of him and watched cars go down into the water.
Traffic was bumper-to-bumper at the time, and he estimated hundreds of people must have been involved.
"It didn't seem like it was real," he said.
Sarah Fahnhorst, who lives in an apartment a block away, heard a huge thud and then "the entire building shook. It shook the ground," she said.
Among the missing was a worker carrying out repairs to the bridge, US media reported. - AFP/ac