Washington, D.C. ? Steel plates connecting steel beams in the Interstate 35-W bridge in Minneapolis were too thin by half and fractured, “the critical factor” in the collapse that killed 13 people and injured 145, the National Transportation Safety Board said Tuesday.

The connectors, called gusset plates, were roughly half the 1-inch thickness they should have been because of a design error, NTSB Chairman Mark Rosenker said. Investigators found 16 fractured gusset plates from the bridge’s center span.

“It is the undersizing of the design which we believe is the critical factor here. It is the critical factor that began the process of this collapse. That’s what failed,” Rosenker said at a news conference.

What caused the bridge to collapse during rush-hour traffic in the early evening of Aug. 1 – “the straw that broke the camel’s back,” as Rosenker put it – was not yet known, he said. A final report by the NTSB was expected this fall.

The Minneapolis span was a steel-deck truss bridge that opened in 1967. Rosenker said it wasn’t clear how the design flaw made it into the bridge because investigators couldn’t find the design calculations.

Rosenker said the safety board had no evidence that the deficiencies in the Minneapolis bridge design “are widespread or go beyond this bridge.”

However, the NTSB couldn’t discount the possibility of similar errors in other like bridges, he said, and cautioned that states and contractors should look at the original design calculations for such bridges before they undertake “future operational changes.”