(CNN) -- At least three people were killed when an interstate bridge in Minneapolis, Minnesota, collapsed Wednesday evening, plunging cars and chunks of concrete into the Mississippi River below.
There were "lots" of injuries, said the state Homeland Security and Emergency Management Department.
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.
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."
Shortly after the collapse, a tractor trailer was burning on the bridge.
Rescue workers are using boats to help remove people from the water, bringing them up on the river bank.
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.
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.
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.
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."
He said there had been construction work on the bridge in recent weeks.
Nighttime 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.
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