Arson is suspected in a six-alarm fire that virtually destroyed a heritage building at 335 Yonge St. on Monday.

“Arson is being investigated as a cause but it is not confirmed to be arson yet,’’ said police spokesman Const. Scott Mills.

Toronto police are working with the Ontario Fire Marshal’s Office to probe the fire that destroyed most of the historic structure at Yonge and Gould Sts.

The façade left standing by the fire, the last charred red brick remnants of the building built in 1888 that once housed the Empress Hotel, will be demolished.

“The engineers have concluded that, structurally speaking, it cannot be saved,’’ said Bryan Fischer, fire inspection supervisor with the Ontario Fire Marshal’s Office.

He said it’s unknown when façade will come down.

Earlier Wednesday, Toronto Deputy Fire Chief Frank Lamie said what’s left of the building fronting Yonge St. is “very unstable. That is why we started (with equipment) at the back — we didn’t want to disturb it.”

Area councillor Kristyn Wong-Tam (Ward 27) had hoped the façade could be saved and had brought a heritage architect to the site Tuesday to inspect it. She said he concluded that due to the extreme conditions it had been subjected to — the flames, the cold water used to extinguish them and the freezing conditions — “the brick had been compromised.

“It would really be unsafe to even try to restore it and bring it back. Only then did I finally give up on trying to save the façade,’’ said Wong-Tam.

Fire investigators still can’t step on the site as it remains unsafe but machinery is being used in the investigation to pull out pieces of debris.

“Heavy equipment is nibbling away at building material and evidence and bringing it out and they’re looking at it,’’ said Fire Capt. Mike Strapko.

According to Fischer, “the fire department has not been able to complete a primary search yet. So that is something that has to be done and will be done as the scene becomes safe to enter.’’

The primary search will determine if any lives were lost. “The evidence we have currently is that there is no one in the building,’’ said Fischer.

On Tuesday, he speculated homeless people could have been inside.

Water was being sprayed on the site Wednesday to “keep the dust down’’ as the building was suspected to have contained asbestos, Fischer said.

He said the investigation was moving along well “but there’s too many variables to comment on how long it’s going to take.’’

Lamie had speculated that “the soonest we can open Yonge St. (to traffic) is Saturday.” But he said that was not a firm date.

Traffic has been diverted and pedestrians are permitted only on parts of Yonge St., between Dundas and Gerrard Sts.

The site is owned by a numbered company linked to the Lalani family.