Six people were killed and five injured when a fire broke out on a bus in Kerzers in the canton of Fribourg on Tuesday afternoon, according to Fribourg police.
Three of the injured are in critical condition. A paramedic who intervened to provide assistance was amongst those injured, along with four passengers.
In an interview on Wednesday morning with Swiss national broadcaster RTS, Fribourg Canton police communications chief Martial Pugin confirmed that while "an intentional act is the most likely scenario", "at present there is no evidence" it was a terror attack.
Videos circulating on social media showed flames several metres high bursting out from the windows, and black smoke rising into the sky.
The area was cordoned off on Tuesday and police asked the public to avoid the site. An investigation has been opened to determine whether the fire was started by a passenger.
Investigators are seeking additional witnesses who can provide information about the moments before the fire began.
It is the second major multiple-casualty fire in Switzerland in less than three months.
In the early hours of 1 January, a bar in the ski resort of Crans-Montana caught fire as people celebrated the New Year. A total of 41 people died, with another 115 injured.
"I am shocked and saddened that people in Switzerland have once again lost their lives in a serious fire," Swiss President Guy Parmelin said on X.
"The circumstances are being investigated. I would like to express my condolences to the families of those who died in Kerzers. My thoughts are also with the injured and the emergency services."
The bus involved was a PostBus, a feature of Swiss rural life.
The distinctive yellow buses serve more remote areas, connecting them with towns, while carrying letters and parcels.
They are used by around half a million people every day, notably schoolchildren.
"This tragic news deeply affects us all. Our thoughts are with the injured and the families of the deceased. A psychological support team will be available to staff" as of Wednesday, PostBus said, according to Switzerland's Keystone-ATS news agency.
