They were looking for causes of a blaze that engulfed an entire block, scorching a strip mall with about a dozen businesses and a half-dozen apartments.
The fire burned down an abandoned grocery store, shops and eateries in St-Donat, a small town in the Laurentian mountains northwest of Montreal.
No injuries were reported.
The fire was already quelled Monday as crews picked through the debris and doused a few smouldering hot spots.
Town residents gathered near the site and took pictures of the blaze, the latest of several major fires to hit the town over the last three decades.
Police were investigating but had little to say Monday about a possible cause.
"We have a couple of investigators specialized in fires that are on the scene right now trying to understand what happened and what led to the fire," said Sgt. Benoit Richard, a provincial police spokesman.
Upon its eruption the previous afternoon, the powerful fire forced people to flee from nearby buildings. Firefighters from the town had to call in reinforcements from five other municipalities and 90 of them worked into Monday to control the flames.
The fire may have started behind an old commercial building, but investigators were only beginning to sift through the site, Richard said.
Mayor Richard Benard cautioned against speculation, calling on anyone who might have seen something to go to police.
"I would like to invite people to be prudent," Benard told a Monday afternoon news conference. "People saw all sorts of things so those people will have to talk to the police."
The burnt building contained an old grocery store that had not been used for about a decade.
Benard said the property was purchased by the municipality a few years ago and there were plans to give it a new function.
Plans to develop the spot will not be abandoned, he said.
"We will make sure there's a project at that spot, regardless," Benard told reporters.
Other businesses that were open when the fire began were destroyed. Benard said they'd like to find new locations for businesses that were destroyed in the blaze.
Benard said plans were under way to clean up the main drag and reopen it to the public later this week.
While it wants to help rebuild the area, the town said it's too early to discuss a time frame. The first step will be to secure and clean the site once police are done with it.
St-Donat's downtown area has been hit by fire several times, the latest incident occurring in January 2000.