The Ontario Fire Marshal is investigating the possibility that fireworks led to the early morning barn fire that left 16 horses dead and forced the rescue of several more on Monday.

A day after the devastating blaze, the cause is still unknown.

However, Toronto Fire Chief Matthew Pegg told reporters that there is an initial report of fireworks in the area Sunday night and early Monday.

"That is something that the Office of the Fire Marshal and police are aware of," he said on Monday.

Heavy equipment was brought in Tuesday to remove debris and take apart the burnt-out structure. (CBC)

Investigators spent the day Tuesday examining the scene, looking for what they describe as "burn patterns".

Pegg said it's a long, slow process.

"We procured and brought in heavy equipment this morning. They will methodically and systematically start to remove some debris and take the building apart as the investigators are looking specifically at burn patterns," he told CBC Toronto.

"We may well be there into tomorrow before they complete the scene examination. From an investigative standpoint, it's a big scene, it's a big structure, it's a long, slow go but we are just working through that process."

Toronto Fire Chief Matthew Pegg says the scene examination is likely to continue until Wednesday. (CBC)

Firefighters were alerted by a resident of a nearby apartment building.

When they arrived at the Sunnybrook Stables at about 3 a.m., firefighters say they found one barn fully engulfed in flames, and another starting to burn.

Thirteen horses survived, and some were taken to the stables at Exhibition Place.

In a Facebook post shared by Ontario Equestrian, the province's athletic association, Sue Iwan, a coach at the stables said the surviving horses have been checked by a vet and are doing well.