If those who designed the bridge in 1964 miscalculated the loads and used metal parts that were too weak for the job, it would recast the national debate that has emerged since the collapse a week ago, about whether enough attention has been paid to maintenance, and raises the possibility that the bridge was structurally deficient from the day it opened. It does not explain, however, why the bridge stood for 40 years before collapsing.

In an announcement, the safety board said its investigators were “verifying the loads and stresses” on the plates as well as checking what they were made of and how strong they were.

State authorities here said the plates were made of steel, and were, in most such bridges, shaped like squares, 5 feet by 5 feet, and a half-inch thick. Such plates are common in bridge construction as a way to attach several girders together, said Jan Achenbach, an expert in testing metals at the Northwestern University Center for Quality Engineering and Failure Prevention.

A consultant hired by the state of Minnesota in the days after the collapse to conduct an investigation of what had gone wrong, even as the national safety board did its work, first discovered the potential flaw, the board said. Representatives at Wiss, Janney, Elstner Associates Inc., the consulting firm, could not be reached late today.

Federal authorities indicated that one added stress on the gusset plates may have been the weight of construction equipment and nearly 100 tons of gravel on the bridge, where maintenance work was proceeding when the collapse occurred. A construction crew had removed part of the deck with 45-pound jack hammers, in preparation for replacing the 2-inch top layer, and that may also have altered the stresses on the bridge, some experts said.

The chairman of the National Transportation Safety Board, Mark V. Rosenker, said Sunday that investigators were calculating the stresses generated on each girder and other bridge components from the construction equipment and materials.

While cautioning other states today about the weight of construction equipment and materials, the federal Transportation Department did not immediately issue any broader warnings about gusset plates. Brian Turmail, a spokesman for the Transportation Department, said this evening that his agency was “conducting additional analysis to determine whether we need to ask the states to do checks of their designs.”