Investigators have not ruled out arson in a massive fire at a building in downtown Toronto on Thursday night.

The vacant building at Parliament and Wellesley streets caught fire after 7 p.m. and erupted into a three-alarm blaze.

Neighbouring buildings were evacuated as a precaution.

Officials believe the fire started in a car parked at the rear of the building.

The building was reportedly being used as a crack house.

The incident is being treated as suspicious.

Officials with the Ontario Fire Marshal's office are investigating.