MINNEAPOLIS, Minn. --- Old photos of the Interstate 35W bridge, featured in a report issued this month by the National Transportation Safety Board, reveal that two steel connecting plates were visibly bent as early as four years before the bridge collapsed and killed 13 people.

The slight bends are visible in gusset plates that hold beams together at two separate connecting points, the Associated Press reported. These plates are located in areas believed to be among the first points of bridge failure at the time of the collapse.

The NTSB's Office of Highway Safety has confirmed that the bowing is a part of the investigation into the causes of the bridge collapse on Aug. 1, the Star Tribune reported. Inspection records failed to mention repairs to the bending gusset plates.

Officials believe the photos were shot by URS Inc., a San Francisco consulting firm that Minnesota hired to examine the bridge from 2003 to 2007, AP reported.