WASHINGTON — 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.

On Aug. 1, 2007, under stress from 287 tons of construction material and rush-hour traffic, the bridge's center span shuddered, then collapsed, dragging other spans into the river. Thirteen people were killed and 145 were injured.

One of the chief lessons of the tragedy, board members said Friday, is that state transportation officials may not probe deeply enough into the design details of their bridges.

"As the stewards of the public dollar, it's the obligation of government to trust but verify," board member Debbie Hersman said.

Over the last four decades, the NTSB has investigated 24 bridge collapses, only five of which resulted from something other than a ship or a truck running into the bridge.

Design flaws, however, may have occurred more often than thought before the Minneapolis bridge collapse. After the tragedy, safety investigators asked transportation officials in 14 states if they had found instances in which bridges had design flaws. Ten states responded they had found at least one.