SEATTLE (Reuters) - The front axle of an amphibious tour vehicle that collided with a bus in Seattle, killing four people, was found to have been sheared off but it was unclear how that occurred, a federal official said on Saturday.

The deadly collision last Thursday on Seattle's busy Aurora Bridge sent about 50 people to area hospitals, officials said.

Soon after the crash, the police said there were witness reports of a possible mechanical issue on the Ride the Ducks amphibious bus that crashed into a charter bus carrying international students.

The collision has placed the operation of duck boats, which carry tourists and have been involved in a number of deadly collisions over the past several years, under increased scrutiny.

The National Transportation Safety Board (NTSB) spent its first full day probing the crash on Saturday, and its investigators were expected to be on scene for at least seven days.

"It's way too early to say anything about probable cause," NTSB member Earl Weener said at a news conference.

The left front axle of the amphibious vehicle was sheared off, but investigators do not know what caused that to happen, Mr Weener said.

National Transportation Safety Board investigators will on Sunday begin interviewing passengers, Mr Weener said.

The crash left the charter bus caved in with glass and debris on the ground.

About 45 students and employees from North Seattle College's international programmes were on the bus. They were headed to a downtown stadium as part of a new student orientation program.

The four killed were college students Claudia Derschmidt, 49, of Austria; Privando Putradanto, 18, of Indonesia; Mami Sato, 36, from Japan; and Runjie Song, 17, from China, North Seattle College said in a statement.

Fifteen people injured in the crash were still being cared for at Harborview Medical Center on Saturday and one was in critical condition, Ms Susan Gregg, a spokesman for the Seattle hospital, said in a statement.

The accident comes nearly five months after an amphibious sightseeing vehicle hit and killed a woman on a Philadelphia street. In 2010, two tourists were killed when a tugboat pushed a barge into a similar vehicle, also in Philadelphia.