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.
Six people are unaccounted for and a number of vehicles were detected in the water.
Officials said the ship suffered a "power issue" and issued a distress call moments before the crash.
Boats and helicopters are part of a huge search and rescue effort searching for the six missing people - down from a previous report of seven.
Authorities said they were part of a construction crew repairing potholes at the time the bridge snapped.
Two others have been pulled from the water so far, one is in a serious condition.
The Singapore-flagged container ship, Dali, struck a support 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 US federal government agency said the vessel "lost propulsion" as it was leaving the port.
The crew then warned Maryland transport officials of a possible collision, according to an unclassified Cybersecurity and Infrastructure Security Agency report on the incident.
Maryland state governor Wes Moore said he could confirm that "the crew notified authorities of a power issue," adding that the ship lost power before smashing into one of the columns supporting the bridge.
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."
Maryland Transportation Secretary Paul Wiedefeld said engineers were at the site determining the structural impact of the collapse.
Shipping company Synergy Marine Group told the OceanNewsUK 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.
"The exact cause of the incident is yet to be determined" and the ship owner was "fully co-operating" with federal agencies, the statement added.
Shipping giant Maersk said it had chartered the vessel and was carrying its customers' cargo.
"We are horrified by what has happened in Baltimore, and our thoughts are with all of those affected", it said in a statement. No Maersk crew and personnel were onboard the vessel.
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.
