SEATTLE (AP) -- A "duck boat" tour vehicle and a charter bus carrying foreign students to a college orientation event collided on a busy Seattle bridge Thursday, killing four students and injuring dozens of others.
The crash happened on the Aurora Bridge, which carries one of the city's main north-south highways over a lake. At least two people were in critical condition among the 51 people authorities said were transported to hospitals.
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.
"We didn't see anything. Just the sound. The smell of gas," said Rujia Xie, a 16-year-old from China, who was on the bus carrying about 45 North Seattle College students and staff. The group was headed to Safeco Field as part of new-student orientation programs.
She said glass fell on their faces, and some people jumped from the bus.
Xie held a bag of ice against some bruises on her face as she left the North Seattle College campus, where the less seriously injured people on the bus were taken.
A driver who was behind the duck boat said the tour bus and duck boat were headed in opposite directions. Brad Volm, of Philadelphia, said the amphibious vehicle swerved in front of him. The left front tire of the duck boat appeared to lock up, and the vehicle swerved into the oncoming charter bus, he said.
Witnesses described hearing a loud screech and then seeing injured people lying on the pavement or wandering around in a daze.
Nurse Jahna Dyer said she was walking across the bridge when she came upon the scene, a mess of jumbled metal and glass. Some victims were lying on the road. Others milled about, seemingly in shock and falling down.
Dyer jumped a railing separating the sidewalk from the roadway to help some of the victims.
John Mundell said he was at the south end of the bridge when the crash occurred.
"We could hear the screech and twisted metal. It was surreal," he said, adding he saw what appeared to be a few dozen people on the ground. "I wanted to try to help. I felt helpless."
When emergency crews arrived, "a lot of people were running at them," pleading for help, Seattle fire Lt. Sue Stangl 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.
Company President Brian Tracey said he did not know what happened.
"Our main concern right now is with the families of those hurt and killed," he said.
Seattle Mayor Ed Murray said the National Transportation Safety Board was taking over the investigation and a federal team would arrive Friday.
North Seattle College spokeswoman Melissa Mixon said students and staff from the school's international program were among those injured, and counselors were on hand.
The school confirmed in a statement Thursday evening that the four people killed were foreign students and government officials were trying to contact family members.
Kuen Shouh Wu said his 18-year-old daughter was on the charter bus but was not hurt. He and his daughter, Ming Chao Wu, are from Taiwan, and he is a visiting scholar at the University of Washington. Wu said when he learned of the accident, he came to the school.
"I was scared," he said. "I don't know why it happened."
The bridge was expected to be closed for hours. It has three lanes in each direction and no barrier separating the north and southbound lanes.
The crash occurred near Canlis, one of the city's fanciest restaurants. The restaurant announced it would close for the night in response, and it employees brought food to first responders.
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 downtown Seattle in 2011.
"These are military craft that were never designed to navigate narrow city streets," Bulzomi said Thursday. "This is a business model that requires the driver to be a driver, tour guide and entertainer at the same time."
In 2010, a tugboat-guided barge plowed 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 -- never resurfaced. They were visiting the U.S. through a church exchange program. Their families received a settlement totaling $15 million 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 cellphone 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.
Attorneys for Elizabeth Karnicki's family allege the May 8 accident, which occurred during rush hour, was due in part to "huge blind spots" on the duck boats.
Associated Press writers Gene Johnson, Martha Bellisle, Lisa Baumann and Manuel Valdes contributed to this report.