Four foreign students have died in a horrific crash on a Seattle bridge between a "duck boat" tour vehicle and the charter bus taking them to a college orientation event .
The crash happened on the busy Aurora Bridge, which carries one of the Washington city's main north-south highways over a lake. At least two people were in a critical condition among the 51 taken to hospital.
There was no immediate word about the cause of the crash, which involved a military-style tour bus that can also be operated on water. Initial reports described the accident as a head-on collision.
Three dozen people were on board the duck boat, as well as the driver, who is certified by the US Coast Guard and a licensed commercial driver, company president Brian Tracey said. He said he did not know what happened or caused the crash.
"We will get to the bottom of it," he said. "Our main concern right now is with the families of those hurt and killed."
"We didn't see anything. Just the sound. The smell of gas," said Rujia Xie, 16, from China, who was on the bus carrying about 45 North Seattle College students and staff. The group was heading to Safeco Field as part of new-student orientation programmes.
She said glass fell on their faces, and some people jumped from the bus.
A driver behind the duck boat said the tour bus and duck boat were travelling in opposite directions. Brad Volm, of Philadelphia, said the amphibious vehicle swerved in front of him and its left front tyre appeared to lock up, sending the vehicle into the oncoming charter bus.
Witnesses described hearing a loud screech and then seeing injured people lying on the pavement or wandering around in a daze.
When emergency crews arrived, "a lot of people were running at them" pleading for help, Lieutenant Sue Stangl of the Seattle fire service said.
The amphibious vehicle is operated by a tour company called Ride the Ducks, which offers tours that are known for exuberant drivers and guides who play loud music and quack through speakers as they lead tourists around the city.
Seattle mayor Ed Murray said the National Transportation Safety Board was taking over the crash investigation and Ride the Ducks had voluntarily taken its vehicles off city streets.
Tourists on the duck boat said they were snapping pictures when they were thrown from the vehicle.
Lying in his hospital bed, Tim Gesner, 61, of Orlando, Florida, told The Seattle Times he was standing in the back of the duck boat and trying to take a picture after their tour guide pointed out the view. He felt the vehicle start to fish-tail and the driver said, "Oh, no".
Mr Gesner looked forward and had a clear view of the duck boat veering left, directly into the bus.
"Then next thing was it's like you see in the movies," he said. "I was floating in this surreal world, like I was in slow motion bouncing off of things and just feeling the pain shooting everywhere and then my face slamming against the seat in front of me and then it was quiet. I just turned and looked, and that's when I saw the carnage."
North Seattle College spokeswoman Melissa Mixon said students and staff from the school's international programme were among those injured and counsellors were on hand.
The school said the four people killed were foreign students and government officials were trying to contact family members.
The bridge has three lanes in each direction and no barrier separating the north and southbound lanes.
The safety of the amphibious boats has been questioned before. Lawyer Steve Bulzomi represented a motorcyclist who was run over and dragged by a Ride the Ducks boat that came up behind him at a stoplight in central Seattle in 2011.
"These are military craft that were never designed to navigate narrow city streets," Mr Bulzomi said. "This is a business model that requires the driver to be a driver, tour guide and entertainer at the same time. It asks too much of the driver."
In 2010, a tugboat-guided barge smashed into a duck boat packed with tourists that had stalled in the Delaware River in Philadelphia.
The crash sank the duck boat and sent all 37 people aboard into the river. Two Hungarian students - 16-year-old Dora Schwendtner and 20-year-old Szabolcs Prem, visiting the US through a church exchange programme - never resurfaced.
Their families received a settlement totalling 15 million dollars (£9.8m) after filing wrongful-death lawsuits against the tugboat and tour boat owners.
Tug operator Matt Devlin was sentenced to a year in prison after pleading guilty to involuntary manslaughter. Devlin acknowledged the accident was caused largely by his continuous use of a mobile phone and laptop computer while he was steering the barge.
In July, the family of a woman struck and killed by an amphibious tourist boat in Philadelphia filed a wrongful-death lawsuit.
Lawyers for Elizabeth Karnicki's family claim the May 8 accident, which occurred during the rush hour, was due in part to "huge blind spots" on the duck boats.