GENEVA (AP) — A bus caught fire in a town west of the Swiss capital killing at least six people and three others were hospitalized with severe injuries, police said Tuesday.

Police spokesperson Frederic Papaux of Fribourg canton, or region, said an unspecified “voluntary act” could be the cause of the inferno Tuesday evening in the town of Kerzers, about 25 kilometers (15 miles) west of Bern, the capital.

The regional transport bus is operated by PostBus, which is affiliated with the national postal service. Images from the scene on Swiss media showed flames tearing through the bus.

“After rushing to the scene, rescue teams noted that the vehicle was totally engulfed in flames,” the regional government said in a statement.

Ambulance and helicopter teams ferried three injured people with severe injuries to hospitals, while two others were treated on site, police said.

Papaux said at least six people were killed. An investigation was underway.