Federal investigators blamed the 35W bridge collapse in 2007 on a design flaw dating back to the 1960s, a controversial opinion that downplayed the obvious shortcomings in inspection and maintenance of the aging, fracture-critical bridge. The same agency had a much more pointed conclusion about what knocked down a section of the Interstate 5 bridge over the Skagit River in Washington in May 2013.

The bridge fell after a heavy truck struck an overhead structural member. Even though two other vehicles plunged into the river, no one was killed or seriously injured.

“This costly accident was the result of a series of mistakes that could have been avoided,” Christopher Hart, acting chairman of the National Transportation Safety Board, said in a statement Tuesday. “The recommendations issued by the NTSB highlight the importance of driver awareness and the states’ responsibilities to provide adequate resources about low clearances.”

The NTSB said many factors contributed to the accident: The driver of the escort vehicle ahead of the truck was distracted by a cell-phone conversation and didn't see whether its "height pole" hit the top of the bridge. The trucking company, Mullen Trucking LP, failed to plan the route to avoid low-clearance obstacles. The NTSB also faulted Washington State's automatic granting of permits for oversize loads. Trucking companies merely had to enter their information into a state web site.

The protection of bridges is “too vital of a state concern to leave the responsibility for assessing the risk associated with the transportation of oversize loads entirely with the motor carrier,” the NTSB said. The agency also blamed the state for failing to put up "low clearance" signs.

The NTSB is an agency with an impressive track record of putting its investigative records online. You can read the file of the I-5 bridge here.

 Above: Interstate 5 bridge in the Skagit River on May 24,2013/Associated Press photo by Elaine Thompson

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