The fire started at the Dousman Transport Company location in Merton at 1 p.m.
Many people who live in the town were devastated to see the damage, including Bill and Barbara Hupe who live nearby.
"As we stood along the trail and we could watch the engines coming in, it would be like Vietnam or some place where they were doing bombing, it just tremored terribly," Barbara Hupe said.
She said they watched the fire grow, as black smoke billowed into the sky. She along with many witnesses reported hearing and feeling explosions, likely caused by the tires blowing.
The Waukesha County Sheriff's Department says ATF agents and sheriff's investigators responded Monday to try and determine what caused the blaze. A spokeswoman with the sheriff's said it's unknown if any surveillance video existed.
"I think it should be investigated definitely because I think this is a suspicious fire," Bill Hupe said.
This particular depot of The Dousman Transport Company serves schools in the Arrowhead and North Lake school districts.
A school official who didn't want to speak on camera said despite the extensive damage, bus service was not disrupted Monday as the company brought in replacement buses from other areas.
In all, firefighters from 24 different departments responded and it took several hours to get the fire under control.
Volunteers on Sunday made about 300 sandwiches for the firefighters at the nearby North Lake Elementary School.
A total of 35 buses and a bus barn were destroyed. Authorities say no one was injured.