Horrific details surfaced Monday in Nova Scotia’s mass shooting, including how the madman apparently first killed his ex and her new boyfriend — and dragged a female Mountie from her car before blasting her execution-style.

Police sources told the Toronto Sun that the death toll — now at 18 — may rise since Canadian killer denture-fitter Gabriel Wortman set fire to homes along the way.

“There could be more’’ victims,’’ a source told the Sun. “There were burned homes that have not been searched.’’

Wortman, a local 51-year-old denturist, planned his rampage in intricate detail, apparently starting Saturday night and first taking out his ex and her beau, police sources told the Sun.

The killer wore a Royal Canadian Mounted Police uniform and drove around in a decommissioned cop car for part of his siege, they said.

“He’s been planning this for awhile,’’ a source said.

Wortman, dressed as a RCMP, drove around to local homes in the town of Portapique or pulled victim drivers over before killing them, officials said.

“They would have trusted him,’’ a source said of Wortman’s victims.

Those killed included “a mom and dad [shot dead] in front of their own children,’’ a police source told the Sun.

Portman set fire to some abodes and vehicles as he murdered people, authorities said.

He also set his own house on fire, police said.

RCMP Heidi Stevenson, a 23-year veteran of Canada’s famed police force, at one point rammed her cruiser into Wortman’s vehicle to try to stop him, sources said.

Wortman then “shot her in the chest with a bunch of rounds,’’ a source said.

Then he “pulled her out of the car and executed her point-blank … took her gun and [ammo], burned his own car and took her car,’’ the source said.

The killer was finally cornered at a gas station in a nearby tiny town and whipped out what is believed to have been Stevenson’s gun before being shot dead in a hail of police bullets, a source said.

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