MINNEAPOLIS (AP) _ It was 1990 when the federal government first issued an ominous label for the state's busiest bridge: ``structurally deficient.''

In the ensuing years, inspectors found cracks and corrosion on the Interstate-35W bridge. They stepped up inspections from once every two years to every year, and made what they thought were the necessary repairs. They were convinced that the bridge had no safety issues at all.

Their actions have come under intense scrutiny since the 40-year-old bridge plummeted into the Mississippi River on Wednesday, killing at least four and injuring another 79.

Police said the death count would surely grow because bodies had been spotted in the fast-moving currents. As many as 30 people were still reported missing.

``We have a number of vehicles that are underneath big pieces of concrete, and we do know we have some people in those vehicles,'' Police Chief Tim Dolan said Thursday. ``We know we do have more casualties at the scene.''

The eight-lane I-35W bridge, which carried 141,000 vehicles a day, was in the midst of mostly resurfacing repairs when it buckled during the Wednesday evening rush hour.

Dozens of cars plummeted more than 18 metres into the Mississippi River, some falling on top of one another. A school bus sat on the angled concrete.

Among the missing is Sadiya Sahal, 23, and her 2-year-old daughter, Hanah Mohamed. Sahal, who is five months pregnant, left home at 5:15 p.m. with the toddler in the back seat. She called her family at 5:30 p.m. saying she was stuck in traffic on the bridge, according to Omar Jamal, a spokesman for the family. That was her last phone call.

``Her husband is destroyed. He's in shock,'' Jamal said.

Officials identified the dead as Sherry Engebretsen, 60, of suburban Shoreview; Julia Blackhawk, 32, of Savage; Patrick Holmes, 36, of Moundsview; and Artemio Trinidad-Mena, 29, of Minneapolis.

Ronald Engebretsen said he and his family were trying to come to grips with his wife's death. ``She's a great person. She's a person of great conviction, great integrity, great honesty and great faith in her God,'' he said.

National Transportation Safety Board chairman Mark Rosenker said his investigators got two big breaks Thursday with a surveillance video showing the collapse and a computer program that would analyze how the bridge failed. Those two things would speed their work and allow them to do a smaller reconstruction of part of the bridge span, rather than the whole thing.

Despite the powerful images of devastation from the collapse, some believed the design of the bridge reduced the death toll.

Joseph Schofer, professor of civil and environmental engineering at Northwestern University, said the bridge's underlying arch truss stopped heavy pieces of steel from falling onto vehicles when the cars plunged into the water.

Gov. Tim Pawlenty responded Thursday by ordering an immediate inspection of all bridges in the state with similar designs, but said the state was never warned that the I-35W bridge needed to be closed or immediately repaired.

``There was a view that the bridge was ultimately and eventually going to need to be replaced,'' he said. ``But it appears from the information that we have available that a timeline for that was not immediate or imminent, but more in the future.''

More than 70,000 bridges across the country are rated structurally deficient like the I-35W bridge, and engineers estimate repairing them all would take at least a generation and cost more than $188 billion.

``I think anybody who looks at the national picture, the national statistics and says that we don't have a problem would be naive or misleading the situation,'' Pawlenty said. ``We have a major problem.''

Authorities cautioned not to read too much into the ``structurally deficient'' tag. The designation means some portions of the bridge needed to be scheduled for repair or replacement. It wasn't a candidate for replacement until 2020.

The collapsed bridge is one of 1,160 bridges in that category, which amounts to eight per cent of bridges in the state. Nationally, about 12 per cent of bridges are labelled ``structurally deficient.''

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

State bridge engineer Dan 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.

After a study raised concern about cracks, the state was given two alternatives: Add steel plates to reinforce critical parts or conduct a thorough inspection of certain areas to see if there were additional cracks. They chose the inspection route, beginning that examination in May.

``We thought we had done all we could,'' Dorgan told reporters near the mangled remains of the span. ``Obviously something went terribly wrong.''

The collapsed bridge's last full inspection was completed June 15, 2006. The report shows previous inspectors' notations of fatigue cracks in the spans approaching the river, including one four feet long that was reinforced with bolted plates.

Although concern was raised about cracks, some experts theorized it's no coincidence the collapse happened when workers and heavy equipment were on the bridge. The construction work involved resurfacing and maintenance on guardrails and lights, among other repairs.

``I would be stunned if this didn't have something to do with the construction project,'' said David Schulz, director of the Infrastructure Technology Institute at Northwestern University. ``I think it's a major factor.''