Old photos of the Interstate 35W bridge show two steel connecting plates were visibly bent as early as 2003 — four years before the span collapsed into the Mississippi River, killing 13 people.
Minnesota Department of Transportation officials declined to say when the state first knew about the bending in the pieces of steel, called gusset plates.
Two photos, part of a report issued earlier this month by the National Transportation Safety Board, reveal slight bends in gusset plates that hold beams together at two separate connecting points. The plates are in areas believed to be among the first points of failure when the span collapsed.
The NTSB's Office of Highway Safety confirmed that the bowing is part of the investigation into why the bridge collapsed Aug. 1, the Star Tribune newspaper reported Sunday.
NTSB Chairman Mark Rosenker didn't comment on the photos, but has said the original design for the bridge specified steel for those and other gusset plates that was too thin.
NTSB spokesman Terry Williams told the Star Tribune the bowing is among "the many things that we are looking at as part of this investigation."
The newspaper said inspection records make no mention of repairs to the bending gusset plates.
Williams did not immediately respond to an Associated phone message seeking comment Sunday. State transportation department spokeswoman Lucy Kender also did not immediately respond to phone and e-mail messages.
The two photos are believed to have been taken by URS Inc., a San Francisco consulting firm the state hired to examine the bridge from 2003 to 2007.
"URS and the state have both got a lot of explaining to do as far as why (the bending) was not observed, and if it was observed, why that was not immediately investigated," James Schwebel, an attorney representing a group of victims, told the AP on Sunday. "How could it possibly have been missed?"
No lawsuit has been filed, but Schwebel said his engineering experts are studying the information.
Attorney Phil Sieff, who represents another group of plaintiffs, said the deformation could have affected how much weight the gusset plate could hold.
Since the bridge's construction during the 1960s, the state highway department had increased weight on the bridge by adding a layer of concrete to the deck in 1977 and by installing concrete barriers in 1998. And the NTSB said last week that, at the time of the collapse, more than 191 tons of construction material had been piled over the bridge's weakest areas.