A tour bus carrying 45 Dutch passengers crashed and overturned in eastern Switzerland, killing five people and injuring 40 others.

The accident happened Thursday in a construction zone between the villages of Susch and Zernez after the bus struck a roadside barrier.

Three of the injured passengers remain in serious condition, while others suffered moderate or minor injuries.

Seven rescue helicopters and 13 ambulance crews responded to the crash and transported victims to nearby hospitals, ABC News has reported.

Swiss authorities are investigating the cause of the crash, and no other vehicles are believed to have been involved.