Sault Ste. Marie Fire Services spent much of the night at a tricky house fire at 50 Wellington Street West.
When firefighters were first called they saw light smoke in the area but no obvious fire.
Subsequent investigation revealed a hot spot in one of the house's walls.
When the wall was opened, considerable smoke was found. When firefighters entered an upstairs attic sometime later they encountered open flames.
Four people were displaced by the fire and Red Cross was called in to help them.
City police remained at the scene at 9 a.m. Sunday.
Power was cut to both the house and the Sault Ste. Marie International Karate Daigaku, an adjacent karate studio, and emergency services tape remained around both buildings.