By Eric M. Johnson

SEATTLE (Reuters) -- Four people were killed and 15 critically injured on Thursday when an amphibious tour bus and a charter bus carrying foreign college students collided head-on on a Seattle bridge, authorities said.

Seattle Mayor Ed Murray said 51 people were treated at area hospitals after the crash on the busy Aurora Bridge, which crosses a canal linking Lake Union with the Puget Sound's Shilshole Bay.

Local TV footage showed the side of the charter bus caved in with glass and debris on the ground. The Ride the Ducks amphibious bus, which had less damage, was missing a front wheel.

Investigators are reviewing witness reports of "a possible mechanical issue" on the duck vehicle, police said.

North Seattle College said in a statement that approximately 45 students and employees from its international programs were on the charter bus. The college said it did not know the exact number of injured or the severity of their injuries.

The college did not say what countries the students were from.

Murray said officials were contacting the consulates of the foreign students.

A witness to the crash, Jesse Christenson, 32, saw the duck boat with its turn signal on trying to get in the left lane, and then it made a sharp move to the left.

"I initially thought it was a (tire) blow out. The duck boat then hit another car with a roof rack then went head on into the oncoming tour bus," Christenson told the Seattle Times newspaper. "The scene was pretty gruesome ... There were people in shock."

Traffic safety on the bridge, which has no median barrier, has been a concern of state and local officials.

Four people died at the scene, authorities said. Among the 51 injured, 15 were in critical condition.

The people on the charter bus suffered the majority of the significant injuries, a fire department spokeswoman said. Two other vehicles were involved in the crash, she said.

"Our thoughts and prayers are with the families involved in today's tragic accident," Ride the Ducks said in a statement.

The National Transportation Safety Board said it will investigate the crash.

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 Ride the Ducks vehicle, also in Philadelphia.

(Reporting by Eric M. Johnson; Additional reporting by Bryan Cohen in Seattle, Dan Whitcomb in Los Angeles and Frank McGurty in New York; Editing by Daniel Wallis andEric Beech)