Michel Huguenot, fire chief for the City of Longueuil, says traces of accelerants were found at the site of the blaze.
Some 70 firefighters were used to put out the flames, which broke out at around 6:30 a.m. on Friday while many residents were still inside.
Three people suffered minor injuries in the fire, including one firefighter.
The investigation has been handed over to police.
A local chapter of the Red Cross is helping families who lost their homes.