CNN) -- At least seven people were killed when an interstate bridge in Minneapolis, Minnesota, collapsed Wednesday evening, in what the governor called a "catastrophe of historic proportions."

The accident occurred shortly after 6 p.m. (7 p.m. ET) when dozens of cars were on the bridge.

Witnesses described a "dust cloud" as it collapsed, sending cars and and chunks of concrete plunging into the Mississippi River below.

All survivors have been taken off the bridge, said Minneapolis police Chief Tim Dolan during a news conference Wednesday night. Crews have searched 50 cars and will be looking for more possible victims, but the mission has changed from a rescue to a recovery mode.

"We have confirmed that this will be a very tragic night when it is over," said Minneapolis Mayor R.T. Rybak.

Recovery operations have ended for the night because conditions became dangerous for workers as darkness set in. "There's too much debris in the river to continue in the river tonight. We will at first light be back in the river working again," Clack said.