(CNN) -- At least six people were killed when an interstate bridge in Minneapolis, Minnesota, collapsed Wednesday evening, plunging cars and chunks of concrete into the Mississippi River below.

Mark Lacroix photographed the collapsed bridge from his apartment window.

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There were "lots" of injuries, the state Homeland Security and Emergency Management Department said.

Local hospitals put the number of people hurt at 37 so far.

The accident occurred shortly after 6 p.m. (7 p.m. ET). There were 50 to 100 cars on the bridge at the time, according to early estimates. Witnesses described a "dust cloud" as the bridge collapsed. See photos of the disaster »

Lt. Amelia Huffman of the Minneapolis Police Department told CNN affiliate KARE it was "not clear at this point what caused the collapse" of the Interstate 35W bridge near University Avenue.

"We have personnel there in the rescue effort," she said. "I have never seen anything remotely like this before."

Mark Lacroix, who lives on the 20th floor of an apartment building near the bridge, told CNN he saw the last seconds of the collapse.

"I heard this massive rumbling and shaking basically and looked out my window," Lacroix said. "It just fell right into the river." Watch Lacroix describe the collapse »

He said there had been construction work on the bridge in recent weeks.

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Construction took place on the bridge Tuesday night and was to take place again Wednesday night, according to the Minnesota Department of Transportation.

The highway would have been restricted to a single lane in both directions from 8 p.m. to 5 a.m. both nights.

The bridge was undergoing redecking work, but nothing structural was being done, U.S. Transportation Department spokesman Brian Turmail said.

The U.S. Department of Homeland Security had no indication that terrorism played a role in the disaster, Sen. Norm Coleman, R-Minnesota, told CNN.

Coleman said Gov. Tim Pawlenty had told him the 40-year-old bridge was inspected and "given a clean bill of health" three years ago.

The nearby University of Minnesota Medical Center received "just a handful" of injuries from the accident, spokesman Ryan Davenport said.

"One of our hospitals has five patients so far, and the other on the other side of the river has none," he said.

Nancy Ebert of Northwestern Hospital said it had received four injured people -- two children and two adults.

Dr. Joseph Clinton, chief of emergency medicine at Hennepin County Medical Center, said the hospital is treating 28 injured people, six of them critically hurt.

He also said the hospital received one patient who was pronounced dead on arrival. "We have one drowning victim here, and I believe there are more drowning victims at the scene," he said.

Two hours after the collapse, a tractor trailer was still burning on the bridge and fire officials were attempting to put out the flames. The vehicle had been cut in half, said Kristi Rollwagen, a spokeswoman for the Minnesota Department of Homeland Security and Emergency Management.

Crews have been using boats to help remove people from the water, bringing them up on the river bank, but bad weather moving into the area could hamper the rescue efforts.

"I don't know how much more could go bad here, but right now, we've got the perfect storm brewing out there, so we're trying to work as hard as we can to pull people out of there" Rollwagen said.

Witnesses told CNN a school bus filled with children was on the bridge when it collapsed, but they also said the bus did not drop into the water and it appeared that the children had all been evacuated.

Aerial footage showed the middle of the bridge caved in, lying in the Mississippi River, with cars both on top and submerged in the water. The main part of the collapsed span is not submerged, but the span clearly separated from the land-based sections of the highway on both the north and south ends of the bridge.

A witness said it looked like "toy cars" were plunging into the water.

"I heard a terrible noise, and then I looked. It seemed like a piece of the bridge was pancaking and going down," said Janet Stately. "I said, 'Did we really see that? Did we really see that?' and it was unbelievable."

About 100,000 cars a day travel over the bridge, according to the Minnesota Department of Transportation.

 

 

I don't want to say anything right now being critical of anybody or anything, I just want to send my condolences to those affected by this disaster.  My thoughts are with you.