Washington ? Undersized steel reinforcing plates were cited Thursday as the chief cause of last year’s deadly collapse of a highway bridge in Minneapolis.

Federal investigators also said the plates were overstressed by almost 300 tons of construction material piled on the bridge.

Investigators told the National Transportation Safety Board that the collapse of the Interstate 35W bridge on Aug. 1, 2007, was unavoidable once gusset plates in the center span failed.

When that happened, it dragged other sections of the bridge and rush-hour commuters into the Mississippi River, killing 13 and injuring 145. The plates are commonly fused to intersecting beams to reinforce the connection.

Investigators focused on the U-10 gusset plates, which were designed at only half the required thickness.

“Had the gusset plates been properly sized, this bridge would still be there,” said Bruce Magladry, director of the NTSB’s office of highway safety.