Swiss authorities say the suspected perpetrator, a 65-year-old Swiss man who one official described as "disturbed", was among the dead, and had earlier been reported missing by his family.
The bus was engulfed in flames on Tuesday evening, local time, on a road near the centre of Kerzers in Fribourg. Kerzers is a town of about 5000 located some 20 kilometres west of the Swiss capital Bern.
"As for the motive, there is absolutely no evidence to suggest that this could be a terrorist act," Fribourg attorney general Raphael Bourquin told a press conference.
A criminal investigation has been opened into suspected homicide, arson, and endangering the lives of others, Bourquin said.
The six dead were formally identified and except for the 63-year-old bus driver who was Portuguese, all of them were Swiss. They included two women, 25 and 39, and two males aged 16 and 29, police said.
The suspect is believed to have boarded the bus with bags before dousing himself in flammable material and starting the blaze, officials said.
Investigators were going through CCTV footage as well as social media posts to understand the man's motives, and said further searches and interviews would take place.
Passengers were seen escaping from the burning bus, panicked and injured. Video after the flames were extinguished showed the charred remains of the yellow vehicle.
Zeynel Teke, 61, was working at his food stall when flames erupted from the bus and it stopped in front of him.
Two or three people came out of the vehicle, and Teke ran to help, using his hand to put out flames on one woman. He went to get his fire extinguisher but the flames were too hot to get close enough to douse them.
"It's so sad to see people burning in front of your eyes. It could be my child, it could be yours," he said.
Two of the injured people remain in hospital, a 56-year-old woman and a 34-year-old man, police said.
Mina Gendre was about to close up the shop she works in when she saw the bus was ablaze inside. "It was so shocking," said Gendre. "I saw someone come running out of the bus on fire."
One video of the fire circulating among Kerzers' residents showed what appeared to be a body in flames moving on the ground next to the burning bus.
Nirosan Vickneswaran, 37, was waiting anxiously for news of his cousin who was on the bus.
"We don't know if he's injured or worse," he said. "All we know is that somebody set fire to themself."
A memorial was quickly erected next to the site of the blaze.
Swiss President Guy Parmelin offered condolences in a post on social media platform X.
"It shocks and saddens me that once again people have lost their lives in a serious fire in Switzerland," he wrote.
In January, Switzerland was rocked by a fire in a bar in the Swiss ski resort of Crans-Montana that killed 41 people and injured 115.
Parmelin laid flowers in Kerzers with local officials on Wednesday afternoon and paid tribute to rescue workers who battled the blaze.
