Calgary fire investigators believe a demolition sparked a three-alarm fire last week at IKO Industries.

The building at the Bonnybrook shingle plant where the fire started is still too unstable and dangerous for investigators to enter.

But through witness accounts and interviews, the fire department is fairly certain the cause was the dismantling of a piece of equipment used to make shingles called "the saturator."

The demolition crew hired to take down the building was using cutting torches and angle grinders to take apart the machine, said fire department spokesperson Carol Henke.

No one was injured in the fire.