The stone was being used to add a two-inch layer to the bridge deck. Its weight, and the weight of the trucks carrying it, would most likely not be a threat to a bridge in good condition, engineers said, but could play a role in causing the failure of a structure that was already weakened.

Completion of the computer analysis is probably still weeks away, investigators said. Once the wreckage is removed from the Mississippi River, the plan is to lay out the debris on shore and identify each piece of metal and its role in the structure, in an effort to determine what failed first.

Investigators will pay special attention to components that had been identified before the collapse as needing repair. The board’s final report on the collapse may be more than a year away.