The Ontario Fire Marshal is investigating a fire in an empty east-end apartment Friday morning which forced the evacuation of a three-storey building.

Police say the tenant was not in their apartment when the fire began, while other tenants were forced to leave the Prospect Street low-rise for the Friday morning blaze. 

There are no damage estimates at this time, although police say the unit in which the blaze began suffered "substantial" damage, while the surrounding units had smoke and water damage.  

Investigators do not yet know the cause of the fire.