At a press conference, police would not be drawn on potential hypotheses including suicide or an act of terror.
In a statement, Fribourg police said a criminal investigation had been opened by public prosecutors “to determine the exact circumstances of the tragedy”.
The burned-out shell of the bus was screened off behind white tarpaulins set up across the road, an AFP journalist saw.
The scene was cordoned off, with police standing guard at the tape. Firefighters and fire trucks were also present.
“Emergency services rushed to the scene and found the vehicle completely engulfed in flames. Firefighters carried out rescue and extinguishing operations. A security perimeter was set up,” the police statement said.
Several ambulances and a helicopter were sent to the scene and three injured people were taken to hospital.
Two other people were examined on site but did not need hospital treatment.
Police said recovery and security operations were ongoing and appealed to the public to stay away from the area.
-Agence France-Presse