SEATTLE — The so-called duck boat was ferrying tourists across a crowded Seattle bridge when, suddenly, the amphibious vehicle swerved into an oncoming charter bus carrying foreign exchange students on their way to an orientation event. The resulting crash killed four people, injured dozens of others and raised safety questions about the distinctive former military vehicles popular with tour groups across the country.

Rujia Xie and other North Seattle College students were on their way to the city’s iconic Pike Place Market and Safeco Field for new student orientation events Thursday when she heard the crash from the back of the bus.

She smelled gas and felt glass falling on her face. She and others jumped from the bus.

Traveling in the opposite direction, two Philadelphia friends on a road trip across the country, Brad Volm and Bradley Sawhill, were cruising over picturesque Lake Union when they said they saw the duck boat’s left tire “lock up” as it swerved into the charter bus, T-boning it. Their SUV hit another truck head-on, but they escaped injuries.

“It all happened so fast. I got out of my car, and there were just bodies, just everywhere. People lying in the street,” Volm said.

The amphibious vehicle is operated by a tour company called Ride the Ducks, which offers tours known for exuberant drivers and guides who play loud music and quack through speakers as they lead tourists around the city.

The collision on the Aurora Bridge, which carries one of the city’s main north-south highways over the lake, left a tangled mess of twisted metal, shattered glass and blood, witnesses said.

North Seattle College issued a statement Thursday evening saying that four of its students died in the crash. Spokeswoman Melissa Mixon said 45 students and staff were on one of two charter buses traveling downtown.

Authorities say 51 people were taken to area hospitals. Susan Gregg, a spokeswoman for Harborview and the University of Washington medical centers, released a statement early Friday that gave the conditions of some of them. At Harborview Medical Center, one person is in critical condition and 11 are in serious condition in intensive care, Gregg said. Three others are in satisfactory condition, she said.

At the University of Washington Medical Center, two people are in serious condition, Gregg said, and two are in satisfactory condition at Northwest Hospital & Medical Center, she said.