A design error was behind the collapse of a road bridge in the US city of Minneapolis that killed 13 people last year, an initial report has said.
More than 100 people were also injured when the I-35W bridge fell into the Mississippi River on 1 August.
Initial findings by the National Transportation Safety Board said metal plates used in construction were not thick enough to bear eventual loads.
It blamed the original 1964 design and not sloppy workmanship.
Relaid concrete
The final report is not due out until later this year.
But the BBC's Vincent Dowd in Washington says the board was keen to make public its initial findings on the metal "gusset plates".
The plates hold huge metal beams in place.
Elsewhere along the bridge the plates were 2.5cm (1in) thick but at the point where they failed they were only half that.
The report did not state outright that the plates caused the collapse but said that over the years the concrete sitting on the steel had been relaid twice, making it 5cm thicker than originally allowed for.
There was no indication of corrosion, other wear or material flaws, the report said.
It said hundreds of bridges across the US now had to be checked to see if similar problems existed.
The board's chairman, Mark Rosenker, said it had been unable to find the original design calculations.
Mr Rosenker told the BBC that road and bridge authorities in other countries needed to consider if the report had lessons outside the US.
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