from the CNN article:
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."