Police had previous knowledge of the Swiss suspect and his profile in a medical context rather than a criminal one, according to Raphaël Bourquin, the regional attorney general. No further details were given.
Philippe Allain, a regional police commander, told reporters that investigators have received information the man had psychological problems.
Identification of the bodies could take days. For locals, the tragedy echoes the New Year’s Eve fire in the Swiss ski resort of Crans-Montana that killed 41 young people.
“It shocks and saddens me that once again people have lost their lives in a serious fire in Switzerland,” President Guy Parmelin wrote on X on Tuesday night. “The circumstances are being investigated. I offer my condolences to the families of those who died in Kerzers. And my thoughts are with the injured and the rescue workers.”
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 engulfing the vehicle.
"We are deeply moved by this tragic news. Our thoughts are with the victims, the injured, and all their families,” PostBus said in a statement on its website.
“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 transported three people with severe injuries to hospitals, while two others were treated at the scene, police said.
The burned-out vehicle was removed during the night and traffic returned to normal as of Wednesday morning.
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Claudia Ciobanu reported from Warsaw, Poland. David Keyton, Kerstin Sopke and Stefanie Dazio contributed to this report from Berlin.