MONTREAL—The condo buildings along the Lachine Canal are new, the grass and flowers are freshly planted, the asphalt on Philippe-Lalonde St. is clean.

What could be responsible for a midnight fire that gutted three condo units? Fire investigators have a theory: someone carelessly smoking on their terrace.

A four-alarm fire broke out in the St-Ambroise area at 12:10 a.m. Wednesday on the third-floor terrace of 5160 Philippe-Lalonde St. With the three-storey condo building connected to its neighbour, the fire department responded quickly with 80 firefighters.

Thirty residents had to be evacuated from the building. No injuries were reported and the fire was under control by about 3:30 a.m.