(CNN) -- Federal regulators said support plates that were about half as thick as they should have been were the likely cause of the August 1, 2007, bridge collapse in Minnesota that killed 13 people and injured 145.

Thirteen people were killed and 145 were injured in the August 1, 2007, Minnesota bridge collapse. more photos »

The gusset plates -- metal plates that are meant to strengthen joists -- are believed to have failed on the I-35W bridge over the Mississippi River in Minneapolis, Minnesota, causing the collapse, according to the National Transportation Safety Board.

There were 111 vehicles on the portion of the bridge that collapsed, dropping them 108 feet into the 15-foot-deep river.

An NTSB report released Friday said the board's investigation found that 24 "under-designed" gusset plates were not discovered in reviews during the bridge's design and construction. The bridge opened in 1967.

Substantial increases in the weight of the bridge because of modifications through the years, along with increased weight on the bridge due to construction equipment and rush-hour traffic the day of the collapse, likely contributed to the faulty plates' collapse, the board said.

The board's investigation determined that other possible reasons for the collapse, including corrosion found on the plates and preexisting cracks in the bridge, did not play a role in the collapse.

"We believe this thorough investigation should put to rest any speculation as to the root cause of this terrible accident and provide a roadmap for improvements to prevent future tragedies," said NTSB acting chairman Mark V. Rosenker. See diagram of bridge »

Don't Miss Victims plan to sue over Minnesota bridge collapse

Several victims of the crash have filed lawsuits against a consulting firm that evaluated the bridge and the construction company whose equipment was on the bridge the day of the collapse.

As a result of its investigation, the safety board made nine recommendations to the Federal Highway Administration and the American Association of State Highway and Transportation Officials dealing with improving how bridges are inspected and how bridge designs are reviewed.

All About I-35W Mississippi River Bridge • U.S. National Transportation Safety Board