SEATTLE – Four college students were killed and at least 51 people were taken to Harborview Medical Center and other hospitals after a charter bus, a "Ride The Ducks" tour vehicle, and two smaller vehicles collided on the Aurora Bridge in Seattle Thursday morning.

At a news conference on Thursday evening, police said the National Transportation Safety Board is taking the lead on the investigation.

"It's too early to draw any conclusions as to cause of this accident," said Seattle Police Chief Kathleen O'Toole.

The bridge was closed for the investigation. The bus and the Ducks vehicle were towed from the scene just before 11 p.m., nearly 12 hours after the crash. The road was reopened shortly after.

Fifteen people remained in critical condition Thursday evening and 17 in serious condition. Harborview officials said ages range from a 17-year-old girl to a 61-year-old man.

Hours after the crash, Harborview Medical Center was still trying to identify three patients. They had no ID and were unconscious or do not speak English.

The charter bus was full of international students and staff from North Seattle College. The buses are owned by Bellair Charters, based in Ferndale.

About 90 Seattle firefighters and medics responded to the scene.

"Seattle had a terrible tragedy today. There has been loss of life and dozens of injuries. Our thoughts and prayers are with the victims, those injured and their loved ones," said Mayor Ed Murray.

Jesse Christenson said he was driving 100-to-150 yards behind the duck, headed northbound.

"The Duck boat was signaling to enter the left lane. As it was making its turn to enter the left lane, it seemed to lurch suddenly. I saw a bunch of smoke and what appeared to be the front, left wheel pop off. It clipped a smaller SUV and basically almost t-boned into the oncoming bus and spun around," said Christenson.

The force was so powerful, a huge gash was ripped in the side of the bus.

Christenson said a flood of people ran up to the scene to help.

Driver Mark Gordon said he was heading south and the Duck boat was heading the other way when the tour boat swerved into his side of traffic

"That's when I turned into oncoming lane of traffic to avoid a direct collision with the Duck," he said.

"The subsequent head-on I had with this smaller SUV and we came to a stop. Really I had little fear left because we were both on our brakes."

When Gordon got out of his truck, he found himself right in the middle of the crash scene.

"I saw people that were literally hanging out the window," he said. "It was total chaos. There were just parts flying, and of course the crash. I got jostled around… finally came to a stop. It was just terrible. There were so many people and so much pain. I could tell what was going on, I just couldn't wrap my mind around it."

Bloodworks Northwest issued an urgent appeal for people to donate blood. Dozens of people lined up to donate.

Ride the Ducks, which operates the amphibious, military-style vehicles, is known for exuberant drivers and tour guides who play loud music and quack through megaphones as they lead tourists around the city. Ride The Ducks says on its website their vehicles have no seat belts. Ride The Ducks announced it was suspending operations.

"It's devastating. The safety of our passengers is our No. 1 concern," said Brian Tracey, Ride The Ducks CEO.

Tracey said the driver is a two-year veteran, and was injured and being treated at the hospital. Seattle Police said a preliminary evaluation of the driver for signs of intoxication did not find any evidence of impairment.

The Utilities and Transportation Commission, which regulates Bellair Charters and Ride the Ducks of Seattle, LLC, said they would be inspecting all vehicle and driver records related to the crash on to determine if the companies complied with state and federal safety regulations.

Phone numbers

North Seattle College established a hotline for people who have questions. That number is 206-934-3672.

Harborview said family members who believe a loved one is at the hospital can call 206-520-5200.

Seattle Police Department has established a hotline for witnesses 206-233-5000.

The Associated Press has contributed to this report.