The landmark Francis Scott Key Bridge in the US city of Baltimore has collapsed after it was hit by a ship.
Several vehicles were crossing the bridge, which is more than 2.6km (1.6 miles) long, when the container ship collided with one of its supports.
At least seven people are believed to be in the water and a number of vehicles were detected beneath the surface, officials said.
Boats and helicopters are part of the search and rescue effort.
The Singapore-flagged container ship, Dali, struck a column on the 47-year-old bridge at 01:30 local time (05:30 GMT), causing it to collapse.
It had departed from the terminal at Port Breeze at around 00:45 bound for the Sri Lankan capital, Colombo.
A number of vehicles, including “one the size of a tractor-trailer” plunged into the water below, officials said.
A major rescue operation is being led by Baltimore fire department, the US Coastguard and other agencies from the state of Maryland.
Baltimore Fire Chief James Wallace confirmed that two people were rescued and one of them had been “transported to a local trauma centre… in a very serious condition”.
Mr Wallace said the tide was posing a challenge to rescue efforts. Local officials said another huge concern for rescue teams was the freezing conditions, with air temperatures of about 3C (37.4F) near the bridge.
Baltimore Mayor Brandon Scott described the incident as an “unthinkable a tragedy”, adding the focus right now should be on “the people, the lives, the souls… there are people in the water that we have to get out and that’s the only thing we should be talking about.”
Reuters A US Coast Guard helicopter flies over the scene of the collisionReuters
Shipping company Synergy Marine Group told the BBC the ship had an all-Indian crew with 22 people on board.
It said in a statement that all crew members – including two harbour pilots – had been accounted for and there were no reports of any injuries.
Shipping giant Maersk said it had chartered the vessel and was carrying its customers’ cargo.
The White House said it was “closely monitoring” the situation and senior administration officials were in touch with the governor of Maryland and mayor of Baltimore to offer any federal assistance they needed, adding there was “no indication of any nefarious intent” behind the crash.
