MINNEAPOLIS — Navy divers cut through tangled debris with underwater torches and saws Wednesday in the search for victims of the collapse of the interstate bridge, while investigators identified a possible flaw in the 40-year-old span's design.
The recovery process has been slowed by huge slabs of steel-reinforced concrete and dangerous chunks of debris submerged in the river's swirling waters.
National Transportation Safety Board investigators said they found a potential design problem with gusset plates -- steel plates that tie together angled steel beams of the bridge's frame.
Investigators are trying to verify loads and stresses on these plates at specific locations as well as the materials used to construct them.
One possible stress may have been the weight of construction equipment and materials on the bridge when it collapsed, the U.S. Transportation Department said in a statement Wednesday.
Safety board and transportation officials said the increased focus on steel plates was preliminary. They would not say exactly where they were located on the bridge or whether their failure alone would have caused the collapse.
"We are continuing to make progress on this investigation, and each area of inquiry gets us closer to ultimately determining the cause of this tragedy," safety board Chairman Mark V. Rosenker said in a statement.
Federal transportation officials were concerned enough with the NTSB finding to issue an advisory to states to "carefully consider" the weight of construction equipment used on bridges.
The Aug. 1 rush-hour collapse of the Interstate 35W bridge hurled vehicles into the Mississippi River 65 feet below, with many tumbling onto the bridge's crumpled concrete deck.
Five people were killed in the bridge collapse, a death toll that was confirmed within a day of the incident. Eight other people are listed as missing.