Bridges collapsing for no immediately apparent reason are extremely rare.

Less rare are collapses with a proximate cause. This spring, an off-ramp of a San Francisco bridge collapsed after it was weakened by a burning gasoline-tank truck. In 2002, 14 people were killed when a barge plowed into a bridge in Oklahoma. In 1980, 35 died when a ship rammed a bridge over Florida's Tampa Bay.

But unique, discrete collapses almost never happen. A notable instance was the collapse in 1980 of the Silver Bridge over the Ohio River with the loss of 46. But the statistical improbability of people suffering the same fate as the motorists caught up in Wednesday's collapse of the Interstate 35W Bridge in Minneapolis is of little comfort when faced with the daily task of getting from one side of the river to the other.

Now engineers must determine why the Interstate bridge failed. It had been built to federal standards, was inspected regularly and was thought to have several years of use left. But it had been identified as "structurally deficient" and may have succumbed to metal fatigue.

Washington was immediately importuned to do something, anything. But after the rescue and recovery operations, the first order of business is to confirm why a bridge undergoing routine use on an ordinary summer day fell without warning. Everything else proceeds from learning that cause.