Investigators have described the main suspect as a gunman with a high-calibre weapon.
Residents on a rural stretch of road reported hearing gun shots at a scrapyard in the town, about 30 kilometres east of Montreal, shortly before 6 p.m. ET.
Just after the gunshots were reported, police cordoned off a wide perimeter near the corner of Des Trente-six and Branche-du-Rapide roads, with about a dozen squad cars and several ambulances visible behind the police tape.
Cécile Boivin lives in the area and heard gunshots at about suppertime Wednesday.
"I heard two bang-bang when I let my dogs out before going to supper at my brother's house in Marieville. Maybe 15 minutes after, I take my car to go to my brother's house and I saw three police cars," Boivin said.
She said she's worried because she has a big barn and vineyard nearby, and fears the suspect or suspects could be in her barn.
Sgt. Ronald McInnis of the Sûreté du Québec said late Wednesday that police wouldn't comment on the nature of the incident while the operation was underway.
Names of the victims haven't been released.