The chair of the National Transportation Safety Board says even now, 12 months after the bridge collapse, investigators are considering all possible causes for the disaster. But Mark Rosenker acknowledges the focus is on the steel connecting plates, called gussets.
"We did, as you know, talk about the potential implications of the gusset plates," said Rosenker. "And also we're looking at aspects of rust, corrosion, that type of thing. But nothing has been ruled out."
Investigators are months from finalizing their report. But last January, Rosenker announced that several gusset plates on the bridge weren't designed properly. They were too thin. From that he was able to point to a potential primary cause.
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"The damage patterns, the fracture features uncovered in the investigation to date suggest that the collapse of the deck truss portion of the bridge was related to the failed gusset plates, and in particular, may have originated with the failure of the gusset plates at one of the eight nodes," he said.
At the time, Rosenker appeared to rule out corrosion as a likely factor behind the collapse. He later backed away from that position.
Then this week, agency investigators revealed they had found that gusset plates had broken into several pieces and were cracked near areas of corrosion. The investigators didn't say whether they thought the gusset plates cracked prior to the collapse, or during the bridge's failure.
MnDOT's own bridge inspections found corrosion back in 1993, and earlier this year the NTSB released old photos of the 35W's bowed gusset plates.
The NTSB's report will be the first of several. MnDOT will release a report on the collapse shortly after the NTSB's report. It will be produced by the consulting firm Wiss Janney Elstner.
PCI, the contractor who was redecking the bridge at the time of the collapse has released a draft report of its findings. The engineering firm, URS, that for years studied the bridge on behalf of MnDOT, is also investigating the disaster.
The NTSB's report is considered the independent, gold standard. Chairman Rosenker says its report will detail the accident, and what he describes as the "probable cause."
"And finally, end with appropriate recommendations to prevent that type of accident from happening again," said Rosenker. "When you say are we going to have the definite moment ... that is still to be determined here. But you will have, and the people of Minnesota will have, a very good, a very thorough understanding of what the probable cause is."
That's what Minnesota U.S. Rep. Jim Oberstar is hoping for. Oberstar, a Democrat, chairs the House Transportation Committee, and he's been critical of the management of this investigation.
"If the board does its job they will cite corrosion, metal fatigue, underdesign of the gusset, failure to conduct periodic, effective inspections of and maintenance of the bridge," Oberstar said.
But others prefer to wait for the NTSB's final report. MnDOT Commissioner Tom Sorel says he has not reached any conclusions about what the NTSB will cite in its investigation. But he acknowledges that today, MnDOT and its bridges have to earn the public's trust.
"We have to be transparent in how we communicate. Work with the Legislature and be transparent with those folks. We got to follow through on what we say we're going to do. We really got to respect public values," said Sorel. "In the end, if we can begin to do all these things I think the trust will come back."
The final report from the NTSB should be out in mid-November. A board hearing will be held in Washington to discuss the findings.