Up to 30 people are missing following Wednesday’s collapse of Minnesota’s busiest bridge. Rescue workers have recovered four bodies, but more are feared trapped inside cars that sunk into the Mississippi River. Seventy-nine people are injured.

Minnesota fire chief Jim Clack: “We’re concerned about the structural stability of the bridge still, the pieces that remain off the ground, so we’re going to be very careful to use the experts to make sure our rescue workers are safe. This is not a rescue operation any longer; it’s a recovery operation. This means we move more slowly and more deliberately.”

The developments come as U.S. officials admitted alarm bells were first raised about the bridge 17 years ago. The Department of Transportation rated the bridge as structurally deficient and began yearly repairs. Two years ago, inspectors gave the same rating and said the bridge would possibly need replacement. More than 70,000 bridges across the country have also been rated structurally deficient.