The report, dated April 28 but not released until this week, does not provide any conclusions but presents several findings, including:
-The west gusset plate at the L-11 node was fractured partially along a line of corrosion. The node - where horizontal, vertical and diagonal beams meet - is near the center of the bridge.
-The east gusset plate at the same node also fractured, with part of the fracture through the line of corrosion.
The report includes a photograph of a gusset plate, with arrows indicating the line of corrosion. The NTSB is still investigating last year's bridge collapse, which killed 13 people and injured 145, with an eye toward coming up with a final cause in 90 to 100 days.
The Star Tribune of Minneapolis first reported the latest NTSB findings on its Web site Tuesday night. According to the newspaper, in 1993, a state bridge inspector discovered that one of the gussets cited in this week's report lost half of its thickness in some spots due to corrosion - but no repairs were ever made.
Minnesota Transportation Commissioner Tom Sorel issued a brief statement saying "some reporters" have drawn "premature conclusions which may be incorrect" from the latest NTSB report.
"MnDOT strongly encourages members of the news media to wait for the NTSB's final report before reporting potentially erroneous theories as fact," Sorel said.
In January, NTSB Chairman Mark Rosenker said that a design error was the "the critical factor" in the collapse, pointing to some gussets - plates that helped connect its steel girders - that were too thin. The NTSB has said that the weight of construction materials on the bridge during resurfacing was also a factor.
The "critical factor" pronouncement had angered some Minnesotans, most notably Democratic Rep. Jim Oberstar, the chairman of the House Transportation Committee. Oberstar said it could commit the board to conclusions before the investigation is complete.
In a telephone interview Wednesday, Oberstar said he wasn't surprised by the new report.
"Corrosion and metal fatigue are factors in bridge failure - they are the enemies of bridge stability," he said. "That's why I was so concerned when Chairman Rosenker pounced rather quickly on the design flaw."
Jim Hall, who was appointed to the NTSB by President Clinton and served as chairman from 1994 to 2001, reviewed the report Wednesday and argued it showed why the board made a mistake when it voted against holding an interim public hearing a few months ago.
"A hearing could have looked at corrosion," he said, adding that an NTSB investigator could use the opportunity to explain how significant the corrosion finding is.
NTSB spokesman Terry Williams repeated the board's position that the time is being better spent preparing for a final report for the investigation. The board will hold a public hearing when it settles on a cause in about three months.
William Schutt, an engineer and corrosion expert who has criticized the status quo of bridge assessment and repair, last year said it was important for investigators to look into corrosion as a cause for the collapse. In a telephone interview Wednesday, he argued that corrosion was the cause of the collapse - not the design of the bridge.
But Joseph Schofer, professor of civil and environmental engineering at Northwestern University, didn't interpret the report that way.
"Corrosion may have encouraged the failure, but this report doesn't read to me like corrosion was the killer," he said.