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Police in January released this image of a 'person of interest' in a downtown alley close to the site of the fire at Yonge and Gould streets. ((Toronto Police Service))

Police have confirmed that arson was the cause of the massive blaze that gutted a historic building at the corner of Yonge and Gould streets in downtown Toronto.

The six-alarm fire began in the early hours of Jan. 3 and laid waste to the heritage building near Ryerson University.

Const. Tony Vella confirmed to CBC News Monday that police have determined the fire was deliberately set.

"The actual reasoning behind that at this point is not going to be released," he said, adding police were still investigating.

"Police are currently working with the [Ontario] Fire Marshal's office. No one has been arrested."

Police had previously said they were investigating the possibility of arson, but had not confirmed the cause until Vella spoke Monday.

In January, they released an image of a "person of interest" in connection to the fire.

The image was taken from surveillance video was provided by a local business owner and shows a hooded person in an alleyway near the scene of the fire.

The person is videotaped heads toward the building around 1:30 a.m., and is seen leaving around 3:50 a.m., police said. The fire was reported shortly after 4 a.m.

Police said it's not possible to tell whether the individual on the video is male or female.