Divers searched the Mississippi River on Thursday for more bodies entombed in cars trapped beneath the twisted steel and concrete slabs of a collapsed bridge. As many as 30 people were missing as the rescue effort shifted to recovery.

The official death count stood at four Thursday morning, but Police Chief Tim Dolan said more victims were still in the water.

“We have a number of vehicles that are underneath big pieces of concrete, and we do know we have some people in those vehicles,” Dolan said Thursday morning. “We know we do have more casualties at the scene.”

The eight-lane Interstate 35W bridge, a major Minneapolis artery, was in the midst of repairs when the bridge buckled during the evening rush hour Wednesday. Dozens of cars plummeted more than 60 feet into the Mississippi River, some falling on top one of another. A school bus hung at an angle on the concrete.

Under water, divers were taking down license plate numbers for authorities to track down their drivers. Getting the vehicles out was expected to take days and involve moving around very large, heavy pieces of bridge.

“The bridge is still shifting,” Dolan said. “We’re dealing with the Mississippi River. We’re dealing with currents. We’re going to have to do it slowly and safely.”

He said police estimate that 20 to 30 people were unaccounted for, though he stressed that it was just an estimate.

Fire Chief Jim Clack said Thursday that emergency work was no longer a rescue operation. “It’s a recovery operation,” he said.

Relatives of some of the missing gathered in a hotel ballroom early Thursday, waiting for any news and hoping for the best.

“I’ve never wanted to see my brother so much in my life,” said Kristi Foster, who went to an information center set up at a Holiday Inn looking for her brother Kirk. She hadn’t had contact with her brother or his girlfriend, Krystle Webb, since the previous night.

Authorities initially said at least seven people had died, but Police Lt. Amelia Huffman lowered that number Thursday morning, saying, “The medical examiner’s office only has four sets of remains.” She said the initial reports were based on the best estimates authorities had Wednesday night.

More than 60 people were injured and as many as 50 vehicles were in the river. Many of their occupants had scrambled to shore after the collapse. Some carried the injured up the riverbank, while emergency workers tended to others on the ground and some jumped into the water to look for survivors. Fire and black smoke rose from the wreckage.

The Homeland Security Department said the collapse did not appear to be terrorism-related, but Hennepin County Sheriff Richard Stanek said Thursday that the cause of the collapse was still unknown.

“All indications are that it was a collapse, not an act of someone doing it,” Stanek said. He said at least a dozen submerged vehicles were visible in the water.

First lady Laura Bush planned to travel to Minneapolis on Friday to console the victims’ families, White House press secretary Tony Snow said.

The bridge had been inspected by the Minnesota Department of Transportation in 2005 and 2006 and no immediate structural problems were noted, Gov. Tim Pawlenty said Wednesday.

“There were some minor things that needed attention,” he said. “They notified us from an engineering standpoint the deck might need to be rehabilitated or replaced in 2020 or beyond.”

A federal database, however, showed the 40-year-old bridge had been rated as “structurally deficient” in 2005 and possibly in need of replacement, the Star Tribune reported citing the U.S. Department of Transportation’s National Bridge Inventory.

“We’ve seen it, and we are very familiar with it,” Jeanne Aamodt, a spokeswoman for the Minnesota Department of Transportation, said of the 2005 assessment. She noted that many other bridges around the country carry the same designation that the I-35W bridge received, and she declined to say what the agency was going to do to address the deficiencies found in 2005.

The bridge was fitted in 2001 with a computerized anti-icing system that sprayed chemicals on the surface during winter weather, according to documents posted on the Minnesota Department of Transportation’s Web site.

This week, road crews had been working on the bridge’s joints, guardrails and lights, with lane closures overnight on Tuesday and Wednesday.

There were 18 construction workers on the bridge at the time of the collapse, said Tom Sloan, head of the bridge division for Progressive Contractors Inc., in St. Michael. One of the workers was unaccounted for.

Sloan said his crew was placing concrete finish on the bridge for what he called a routine resurfacing project. “It was the final item on this phase of the project. Suddenly the bridge gave way,” he said.

Sloan said his workers described a horrific scene. “They said they basically rode the bridge down to the water. They were sliding into cars and cars were sliding into them,” he said.

The entire span of Interstate 35W crumpled into the river below. Some injured people were carried up the riverbank, while emergency workers tended to others on the ground.

The school bus had just crossed the bridge when it collapsed. It did not go into the water, and the children were able to escape unharmed out the back door.

Christine Swift’s 10-year-old daughter, Kaleigh, was on the bus, returning from a field trip to Bunker Hills in Blaine. She said her daughter called her about 6:10 p.m.

“She was screaming, ’The bridge collapsed,”’ Swift said. All the kids got off the bus safely, but about 10 of the children were injured, officials said.

Sen. Norm Coleman, R-Minn., said he spoke with Transportation Secretary Mary Peters, and that both of them along with Sen. Amy Klobuchar, D-Minn., will be flying to the Twin Cities early Thursday.

The collapsed bridge is just blocks from the heart of Minneapolis, near tourist attractions like the new Guthrie Theater and the Stone Arch Bridge. As the steamy night progressed massive crowds of onlookers circulated in the area on foot or bicycle, some of them wearing Twins T-shirts and caps after departing Wednesday night’s game at the nearby Metrodome early.

Thursday’s game between the Twins and Kansas City Royals was called off, but the Twins decided to go ahead with Wednesday’s rather than sending about 25,000 fans back out onto the congested highways. Inside the stadium, there was a moment of silence to honor victims.

The steel-arched bridge, which was built in 1967, rose about 64 feet above the river and stretched about 1,900 feet across the water. The bridge was built with a single 458-foot-long steel arch to avoid putting any piers in the water that might interfere with river navigation.

The river’s depth at the bridge was not immediately available, but the U.S. Army Corps of Engineers maintains a channel depth of at least 9 feet in the Upper Mississippi from Minneapolis southward to allow for barge and other river traffic. The site is just downstream from the St. Anthony Falls locks and dams.