A tour vehicle and a charter bus carrying foreign students have collided in Seattle, Washington state, leaving four people dead, USofficials say.
TV footage showed an amphibious "Ride the Ducks" vehicle - a type used for tours in several US cities - smashed into the side of the bus on a bridge.
The four victims were foreign students from North Seattle College. Several people were critically injured.
Seattle Mayor Ed Murray described the crash as "a terrible tragedy".
It happened at about 11:00 local time (18:00 GMT) on Thursday. Two smaller vehicles were also involved in the crash.
Most of the victims were on board the charter bus. Officials have not said which countries they were from.
Forty-five students and staff members from the school's international programmes were taking an orientation trip ahead of the school year starting on Monday.
Fifty-one people were taken to hospitals. Fire department officials said 12 had critical injuries.
"Today, the North Seattle College community learned devastating news that four of our students were tragically killed in a vehicle accident on the Aurora Bridge," the school said in a statement.
"Additionally, several students remain in critical condition, and other students and a North employee sustained serious injuries."
Government officials were working to contact next of kin.
A witness said the duck boat signalled to move left when it lurched and the front left wheel came off. It then clipped a vehicle before colliding with the bus.
Two witnesses travelling in the opposite direction, Brad Volm and Bradley Sawhill, told the Associated Press news agency they saw the duck boat's left tyre "lock up" as it swerved into the charter bus.
"It all happened so fast. I got out of my car, and there were just bodies, just everywhere. People lying in the street,'' Mr Volm said.
Tim Gesner, who was on the duck boat, told The Seattle Times the vehicle started to fish-tail and he heard the driver say "Oh, no".
Mayor Murray said the company had voluntarily suspended trips for the time being.
"Ride the Ducks" tours, using vehicles which can drive on roads and float on water, are known for guides who play loud music and quack through speakers when leading tourist groups.
The amphibious vehicles have been involved in multiple fatal accidents.
Two people died when one collided with a barge in Philadelphia, Pennsylvania, in 2010.
The "Duck" name is derived from DUKW, the six-wheel amphibious vehicles used by the US military during World War Two.