WASHINGTON—Under-sized 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 Inter-state 35W bridge on

Aug. 1, 2007, was unavoidable once gusset plates in the center span failed.

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.