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An Interstate Highway bridge ends abruptly where it collapsed into the Mississippi River during rush hour traffic in Minneapolis, Minnesota on August 1, 2007. Interstate 35W is a major highway leading out of downtown Minneapolis that has been under construction this summer. (UPI Photo/Wayne Thomas)

Mark Rosenker, the chairman of the National Transportation Safety Board, said at a news conference in Washington that the plates should have been 1-inch thick and instead were one-half-inch thick, the St. Paul Pioneer Press reported. He said that the agency hasn't found a "probable cause" for the collapse but determined the gusset plates were significant.

The bridge over the Mississippi River collapsed during rush hour Aug. 1. Thirteen people died and 144 were injured.

The gusset plates connected beams on the end of the bridge where the collapse started.

Rosenker said that the discovery was "a shock to the investigative team."

"We've never seen anything like this," he said.

Two upgrades also added weight to the bridge, Rosenker said.

Transportation Secretary Mary Peters sent an advisory to states Tuesday urging them to calculate how changes in bridge weight, capacity or conditions could affect the stress on gusset plates.