Detectives investigating the fire at Ferryhill Motors in Torrance, East Dunbartonshire, said it was being treated as wilful and have appealed for witnesses.
As reported in yesterday's Evening Times, residents living nearby were evacuated from their homes as more than 30 firefighters tackled the blaze on Monday night.
Around 70 staff work at the family-run firm in Campsie Road but the premises were closed at the time and no-one was injured in the incident.
Propane gas cylinders stored in the garage exploded due to the heat as firefighters battled to stop the fire from spreading to other vehicles.
The fire destroyed eight buses and six caravans and is estimated to have caused tens of thousands of pounds of damage.
A joint investigation is under way between police and the fire service and detectives are studying CCTV images of the yard in a bid to trace the culprits.
Kieran McCahill, owner of Ferryhill Motors, which has been running for 42 years, said: "The fire brigade did an excellent job in containing the fire but it ended up in a chain reaction with more buses and other vehicles lost.
"Thankfully, there was no damage to the workshop or the surrounding area."
Detective Inspector Sharon MacGregor said: "It's imperative we trace whoever is responsible for setting this fire.
"I would appeal to anyone who may have seen or heard anything suspicious to get in touch with us."
Anyone with information is asked to contact Kirkintilloch Police Office on 0141 532 4400, or Crimestoppers on 0800 555111.