The flames at the former Monastère du Bon-Pasteur building in downtown Montreal have been extinguished, but over a dozen firefighters remained on site early Saturday morning to ensure the fire wouldn't reignite.

Residents were permitted to enter and retrieve their essentials, such as important documents and medications, by the early afternoon.

The five-alarm blaze started Thursday afternoon and burned into Friday evening, placing 59 people into the care of the Red Cross and triggering an air quality advisory due to smoke.

Service de sécurité incendie de Montréal (SIM) chief Richard Liebmann said Friday that the fire was "very difficult" to extinguish due to the building's structure and construction materials.

A man in his 80s was discovered inside hours after other occupants fled. He was sent to hospital to be treated for hypothermia, and SIM authorities say it's unclear why he was overlooked in the evacuation.

The building, a former monastery, was built in 1846 at the corner of Sherbrooke and de Bullion Streets. A recognized heritage site, it housed residential units, a daycare, community organizations, and a concert hall.

It also contained precious historical and cultural items now feared to be destroyed, including a 250-year-old harpsichord and a $300,000 piano.

An investigation into the cause of the fire is set to begin on Monday.

With files from The Canadian Press.