US officials have ordered a review of all bridges similar to the one that collapsed in Minnesota two days ago, killing at least five people.
More than 700 structures will be looked at after it emerged that the bridge in Minneapolis had been classified as "structurally deficient".
Divers have resumed their search for victims in the Mississippi river. At least eight people are still missing.
First Lady Laura Bush has been visiting the scene of the collapse.
Mrs Bush praised rescue workers for their efforts.
"I know that....even though you're trained, it's hard. And I know it's hard for you psychologically as well, to watch and to see the people who were grieving. And I know you grieve with them," she said.
"So I want to thank you very very much for helping all the people here, for serving our country here."
Some 50 vehicles were hurled into the water when part of the I-35W bridge in the city gave way during the rush hour on Wednesday evening.
Local hospitals said they treated 79 people, most of them suffering from broken bones and head and spinal injuries.
Before resuming the search on Friday morning, Hennepin County Sheriff Rich Stanek said whirlpools and currents had hampered the efforts of his divers.
"Conditions on the river (are) even more treacherous than yesterday," he told reporters.
Earlier, Minneapolis Mayor Raymond Thomas Rybak said that "sadly and tragically", he did not believe there would be any more survivors.
"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."
The authorities were quick to rule out terrorism as the cause of the collapse. Attention is focusing on the bridge's state of repair.
Transportation Secretary Mary Peters said all structures similar to the collapsed steel truss bridge in Minneapolis would be inspected thoroughly.
"Even though we don't know what caused this collapse, we want states to immediately and thoroughly examine all similar spans out of an abundance of caution," she said.
Mr 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.
The disaster has focused attention on the country's ageing infrastructure - particularly its bridges.
I-35W BRIDGE 9340
Eight-lane, steel-arch bridge
Built in 1967
Used by 140,000 cars a day
According to the Federal Highway Administration, more than 70,000 bridges in the US - about 12% of all bridges - were classified "structurally deficient" in 2006.
But a bridge may be judged deficient simply for having weight restrictions, bans on heavy vehicles, or if certain parts need repairing.
Another 70,000 bridges have been given an even worse grade - "structurally obsolete" - says our correspondent Kim Ghattas.
The 40-year-old I-35W highway bridge, which was being repaired when it collapsed, was not due for replacement until 2020, said Minnesota Governor Tim Pawlenty.
He expressed concern for the rest of the country.
"Anybody who looks at the national picture, the national statistics and says that we don't have a problem would be naive or misleading the situation. We have a major problem," Mr Pawlenty said.
Experts have estimated the cost of repairing all the nation's bridges at between $55bn (£27.5bn) and $188bn.
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
Democrat Senator Harry Reid said the tragedy should be treated as a "wake-up call".
"We have all over the country a crumbling infrastructure - highways, bridges and dams. We really need to take a hard look at this," he said.
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.
The federal Department of Transportation has offered an initial $5m to help with clean-up efforts and transport issues in Minnesota, and Congress has begun working on legislation to provide $250m to replace the bridge.