Waterloo Regional Police say they have started a formal arson investigation related to the fire which damaged a number of condo units in the area of Louisa Street and St. Leger Street in Kitchener in July.
No one was hurt in the incident, but a number of residents were forced to leave their homes and damage to three blocks of townhouses was estimated at $5 million.
Police spokesperson Olaf Heinzel said police investigators have been involved since the beginning.
"It's only at this point now though where information has been gathered and the ongoing investigation has led investigators to determine that this fire was started."
Heinzel could not provide further information on what led police to believe the fire was a criminal act. Police are currently leading the investigation, and the Ontario Fire Marshal's office is involved as well.