MINNEAPOLIS — Minnesota officials were warned as early as 1990 that the bridge that collapsed into the Mississippi River was "structurally deficient," yet they relied on patchwork repairs and stepped-up inspections that unraveled amid a thunderous plunge of concrete and automobiles.

"We thought we had done all we could," state bridge engineer Dan Dorgan told reporters not far from the mangled remains of the span. "Obviously something went terribly wrong."

Questions about the cause of the collapse and whether it could have been prevented arose Thursday as authorities shifted from rescue efforts to a grim recovery operation, searching for bodies that may be hidden beneath the river's swirling currents.

The official death count from Wednesday's rush-hour collapse stood at four, with another 79 injuries. But police said the death count would surely grow because bodies had been spotted in the water, and as many as 30 people were still reported missing.

The Army Corps of Engineers lowered the river level a foot to help recovery efforts, said agency spokeswoman Shannon Bauer.

In 1990, the federal government gave the I-35W bridge a rating of "structurally deficient," citing significant corrosion in its bearings. The bridge is one of about 77,000 bridges in that category nationwide, 1,160 in Minnesota alone.

The designation means some portions of the bridge needed to be scheduled for repair or replacement, and it was on a schedule for inspection every two years.

Dorgan said the bearings could not have been repaired without jacking up the entire deck of the bridge. Because the bearings were not sliding, inspectors concluded the corrosion was not a major issue.

During the 1990s, later inspections found fatigue cracks and corrosion in the steel around the bridge's joints. Those problems were repaired. Starting in 1993, the state said, the bridge was inspected annually instead of every other year.

A 2005 federal inspection also rated the bridge structurally deficient, giving it a 50 on a scale of 100 for structural stability.

The eight-lane I-35W bridge was Minnesota's busiest bridge, carrying 141,000 vehicles a day. It was in the midst of mostly repaving repairs when it buckled during the evening rush hour. Dozens of cars plummeted more than 60 feet into the Mississippi River, some falling on top one of another. A school bus sat on the angled concrete.

Engineers wondered whether heavy traffic might have contributed to the collapse. Studies of the bridge have raised concern about cracks caused by metal fatigue.