Ending a 15-month investigation, the National Transportation Safety Board has determined that the probable cause of last year's fatal bridge collapse in Minneapolis was the failure of gusset plates due to a design error and the bridge's increased live and dead loads caused by earlier upgrades and the traffic and and construction materials and equipment on its deck at the time it fell.
Thirteen people were killed and 145 injured on Aug. 1, 2007, when most of the 41-year-old bridge fell into the Mississippi River.