Report: Tons of rock, sand piled on bridge just before collapse

• Story Highlights
• I-35W bridge in Minneapolis collapsed on August 1, 2007, killing 13 people
  
• Construction material was piled on weakest parts of bridge, investigators say
  
• Connectors on bridge called gusset plates were too thin, probe finds
  
• National Transportation Safety Board says new info may explain thin gusset plates
  
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