Federal officials cautioned transportation authorities nationwide to re-examine bridges after investigators probing a deadly interstate bridge collapse said they found a potential design flaw.

The National Transportation Safety Board said Wednesday that it found issues with the collapsed bridge's gusset plates -- the steel plates that tie steel beams together.

U.S. Transportation Secretary Mary Peters also advised states to consider the additional stress placed on bridges during construction projects. An 18-person crew was working on the Interstate 35W span when it collapsed last week during evening rush hour, killing at least five people and injuring about 100.

"Given the questions being raised by the NTSB, it is vital that states remain mindful of the extra weight construction projects place on bridges," Peters said Wednesday.

In an update of its work, the NTSB said helicopter observations had found several "tensile fractures" in the superstructure on the north side of the bridge, but nothing that appeared to show where the collapse began. Investigators were verifying loads and stresses on the beams, as well as materials in the plates.

NTSB investigators have also been looking into reports of wobbling before the Aug. 1 collapse.

The company that was doing the construction work, Progressive Contractors Inc., rejected a report that a worker noticed unusual swaying of the bridge in the days before its collapse. The company said it didn't believe any of its work contributed to the bridge failure but hadn't responded directly to claims of wobbling.

"We have now met with every single worker who was on the bridge when it collapsed," Tom Sloan, vice president of the company's bridge division, said in a news release Wednesday. "None of them observed or reported any unusual swaying."

Officials of the Minnesota Department of Transportation wouldn't comment on the significance of the gussets in the bridge's collapse.

Even as the federal warning was issued, Navy divers continued probing the wreckage of the collapsed bridge for bodies, and officials said they expected removal of heavy debris to begin later than expected to give the divers more time.

At least eight people are missing and presumed dead. At least eight more were still hospitalized, one in critical condition.

At the dive site, two large cranes were ready to go. But they sat idle as divers returned to the water doing "a very meticulous, hand-over-hand search of the scene," said their spokesman, Senior Chief Dave Nagle.

Navy and FBI dive teams are trying to go deeper into the debris of the bridge than the local dive teams that have been working since the Aug. 1 collapse, police Capt. Mike Martin said. He expects it to be at least a week before cranes start regularly hauling out large pieces of debris.

The FBI team had to abandon using the larger of its two unmanned submarines. The remote-controlled vehicle -- equipped with a camera, sonar, lights and a grabbing arm -- was too big to maneuver amid the unstable, twisted bridge wreckage and vehicles in the cloudy water, agent Paul McCabe said Wednesday.

Instead, FBI divers will try their smaller sub, a shoe-box-size vehicle equipped only with lights and a camera. It is more difficult for the sub to navigate the Mississippi River's stiff river currents because of the sub's smaller thrusters. The water where the divers are working ranges from 2 to 14 feet deep.

Debris removal had been expected to begin this week. The State Patrol said 88 vehicles have been located at the collapse site, including those in the Mississippi River.
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