Washington, D.C. ? The design flaw that brought down a highway bridge in Minneapolis went undetected for 40 years. Federal safety officials say that, even today, government design reviews might not detect similar problems.

The National Transportation Safety Board made official Friday what has been clear for months – that the failure of undersized, steel gusset plates was the probable reason Interstate 35W bridge buckled and then fell into the Mississippi River.

The board found that engineers who designed the bridge in the 1960s either failed to calculate or improperly calculated the thickness needed for the plates that were to hold the bridge together.

They also blamed state and federal highway officials for not catching the design flaw at the time the bridge was approved for construction.