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Four international college students were killed after the charter bus they were in collided with a "duck tour" vehicle on a Seattle bridge Thursday, officials said.

The crash involving a bus, the duck tour vehicle, and two passenger vehicles occurred at about 11:15 a.m. local time (2:15 ET) on the Aurora Bridge.

Four were dead when firefighters arrived, and 51 were transported to area hospitals, Seattle Fire Chief Harold Scoggins said. Fifteen people were in critical condition Thursday evening, he said.

North Seattle College said Thursday evening that the four killed were international students, and several other students remained in critical condition. A college employee was seriously hurt.

A Seattle official at the scene of a deadly crash involving a "Ride the Ducks" amphibious vehicle, right, and several other vehicles Thursday, Sept. 24, 2015, in Seattle. Ted S. Warren / AP

"No words are adequate to express the absolute pain and sorrow we all feel as a result of receiving this devastating news," the college said in a statement.

A witness who was driving behind the duck tour vehicle told NBC affiliate KING 5 that the amphibious vehicle and the bus were traveling opposite directions when the duck tour vehicle appeared to lose control.

"The duck boat was signaling to enter the left lane, and as it was making its turn to enter the left lane, it seemed to lurch suddenly," Jesse Christenson, who said he was about 150 yards behind the duck boat, told the station.

"I saw a bunch of smoke and what appeared to be the front left wheel pop off, and then it clipped a smaller SUV and basically almost T-boned into the oncoming bus and spun around," he said.

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An Orlando man on board the duck boat told the Seattle Times that the vehicle began to fishtail as passengers were taking a photo of Lake Union as the vehicle crossed the bridge.

"Just at that moment, as it was starting to fishtail, I heard the driver go, "Oh no,'" Tim Gesner, 61, told the paper from his hospital bed. The duck boat then struck two vehicles and the bus.

The crash is still under investigation and a cause has not been determined. The impact ripped a huge gash in the side of the charter bus.

The charter bus was carrying approximately 45 students and staff of North Seattle College's international programs to Safeco Field for an orientation when the accident occurred, the college said.

The names of the students killed in the crash were not released. The school said government officials were still working to notify their families.

"We’ve had a terrible tragedy. There's been a terrible loss of life and there's been injuries," Seattle Mayor Ed Murray said. "Our thoughts, the thoughts and prayers of this city go out to everyone, to the families and to those impacted."

Ride the Ducks' amphibious vehicles give tours of Seattle by boat and by land.

"Ride the Ducks of Seattle wishes to express its sincerest condolences to the family and friends of the people who were killed and those that were injured," the company said in a statement.

Ride the Ducks CEO Brian Tracey said he did not know how many people were on board the duck boat, but he said he assumes it was a at near-capacity, which is 36 passengers and a captain.

Tracey said the driver, who was injured, has been with the company for about two years. All drivers have a Coast guard license and a commercial drivers license, he said. The company said it would suspend tours until Monday out of respect for the victims.

The National Transportation Safety Board said it was sending a team to investigate the crash.

The Washington Utilities and Transportation Commission said it conducted a "comprehensive safety inspection" of the Ride the Ducks fleet in 2012, and the company received a satisfactory rating.

Injured people are taken to an ambulance at the scene of a crash between a tour bus and a tourist duck boat on the Aurora Bridge in Seattle on Sept. 24, 2015. MARK RALSTON / AFP - Getty Images

The Federal Motor Carrier Safety Administration inspected the charter bus company, Bellair Charters, in 2013 and it also received a satisfactory rating, the UTC said.

John Mundell said he was at the south end of the bridge when the crash occurred.

"We could hear the screech and twisted metal," he told The Associated Press. "It was surreal."

Mundell said he saw what appeared to be a few dozen people on the ground.

"I wanted to try to help," he said. "I felt helpless."