Divers in the US city of Minneapolis have suspended their search overnight for victims of a bridge that collapsed over the Mississippi river.
Officials said it was too dangerous for the divers to work in the dark.
Four people are confirmed dead and at least eight people are missing. Police
had earlier said up to 30 were missing.
Some 50 vehicles were hurled into the water when part of the I-35W bridge in Minneapolis, Minnesota, gave way during the rush hour on Wednesday evening.
Local hospitals said they treated 79 people, mainly suffering from broken bones and head and spinal injuries.
Minneapolis Mayor Raymond Thomas Rybak said: "Sadly and tragically, we don't believe there are people who are alive in the river.
"That means we are going to be very, very careful not to risk anything with the divers who are there, so we will be cautious in that regard."
US President George W Bush said the government had to "respond robustly" to help the people of Minnesota recover and rebuild the bridge as quickly as possible.
Mr Bush will travel to the scene of the collapse on Saturday. His wife, Laura Bush, will visit Minneapolis on Friday to meet victims of the disaster.
The collapse has focused US attention on its ageing infrastructure, particularly bridges.
Transportation Secretary Mary Peters called on all states to inspect the more than 700 steel truss bridges across the country similar to the one in Minneapolis.
The federal Department of Transportation has offered an initial $5m (£2.5m) to help with clean-up efforts and transport issues.
Congress has begun working on legislation that would provide $250m to replace the bridge.
The 40-year-old I-35W highway bridge was being repaired at the time of the collapse.
Minnesota Governor Tim Pawlenty ordered an immediate inspection of all bridges in the state with similar designs. He said the bridge was last inspected in 2006 and no significant structural problems were found.
But in 2005, the bridge was one of thousands across the US rated as "structurally deficient" on the federal National Bridge Inventory database.
I-35W BRIDGE 9340
Eight-lane, steel-arch bridge
Built in 1967
Used by 140,000 cars a day
White House press secretary Tony Snow said it was the state's responsibility to correct the problems if a bridge was reported to have deficiencies, even though the inspection did not indicate the Minneapolis bridge was at risk.
Investigators from the National Transport Safety Board (NTSB) would review video of the collapse and recover pieces of the bridge and reassemble them to determine how and why the bridge collapsed, NTSB Chairman Mark Rosenker said.
Traffic had been "bumper to bumper" at the time of the incident as only one lane was open in either direction on the eight-lane road owing to construction work on the arched bridge.
Tons of concrete crashed 64 feet (20m) into the water as the structure crumbled.
Injured people waited for rescue as vehicles dangled off the crumpled concrete and smoke billowed into the sky from the fiery wreckage of a lorry.
Largest city in Minnesota state
It and adjoining state capital St Paul form part of the Twin Cities area
Twin Cities area population about 3.5m
Some people managed to swim to safety.
"The bridge started falling, cars were flying everywhere and I saw the water coming up," Catherine Yankelevich, whose car ended up in the water, told AFP news agency.
At least 50 children were led to safety with only minor injuries from a school bus that clung to the side of a collapsed section over the river.
Eyewitnesses said they heard a huge roar as the structure fell into the river.
"What I first heard was a giant rumbling sound from my apartment," Mark Lacroix told the BBC.
"I'm 20 storeys up, just a stone's throw from the bridge itself, and I thought, maybe my own building was coming down, it was an amazing noise. And I look out of the window and I see just the final moments of it collapsing into the river.
"It really took quite a while for me to realise that this was not a planned explosion... to realise it was a tragedy."