Old photographs of the Interstate 35W bridge in Minneapolis show that two steel connecting plates were visibly bent as early as 2003 — four years before the span collapsed into the Mississippi River, killing 13 people, according to the Associated Press.

Minnesota Department of Transportation officials declined to say when the state first knew about the bending.

Two photographs, part of a report issued earlier this month by the National Transportation Safety Board, reveal slight bends in the two gusset plates, which hold beams together and were in areas thought to be among the first points of failure in the collapse. The board’s Office of Highway Safety confirmed that the bowing was part of the bridge investigation, the Star Tribune newspaper reported Sunday.

Mark V. Rosenker, the chairman of the safety board, did not comment on the photographs, but he has said the original design for the bridge specified steel that was too thin for those and other gusset plates.

A spokesman for the board, Terry Williams, told the Star Tribune that the bowing was among “the many things that we are looking at as part of this investigation.”

The newspaper said inspection records made no mention of repairs to the gusset plates.

Since the bridge’s construction in 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 National Transportation Safety Board said last week that at the time of the collapse more than 99 tons of sand had been piled on the roadway directly over two of the bridge’s weakest points.