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"He tracked the children’s mother, showed up at her house, repeatedly sent her text messages and frequent calls," Cantin wrote in a report released late Monday.

Lamontagne was arrested outside the woman’s residence in August 2023 for criminal harassment, then released on a promise to appear in court at a later date. The report does not mention charges being filed. About two days later, Lamontagne used helium to suffocate three-year-olds Antoine and Tristan Lamontagne at his home in Notre-Dame-des-Prairies, Que., north of Montreal.

Repeated attempts by a potential perpetrator to control a partner or ex-partner are risk factors for domestic violence and homicide, Cantin noted in his report. And he questioned whether a mediator who counselled the family, or the police who responded to the mother's complaint, could have spotted the signs.

"All the elements set out in the criminal harassment complaint in the context of domestic violence filed by the children’s mother were significant risk factors," Cantin wrote. 

"The police could consider these risk factors to better assess the dangerousness of the situation and establish more restrictive conditions to ensure a safety net for the children."

Coroner reports do not assign blame; rather, they make recommendations aimed at preventing future tragedies.

A domestic violence expert told ºÃÉ«tvafter the toddlers' deaths that there are several risk factors for domestic homicide: a couple's separation; escalation of abuse; expressions of fear; and a partner showing signs of depression, suicidal ideation, or obsessive behaviour.

Cantin's report noted that Lamontagne had been treated for depression and anxiety, and had attempted suicide in 2018 and 2022. The report also noted that Lamontagne and his partner had broken up multiple times before separating for good in early 2023.

Following the separation, Lamontagne would call and text his former partner, telling her he knew where she was, the report said. She later found a phone hidden in her car.

The mother called police on Aug. 24 after her neighbours reported seeing Lamontagne regularly driving around her building. Police who arrived to take her statement found Lamontagne near the premises and arrested him. He was released on the condition that he not contact his ex-partner other than to pick up or drop off the children. 

The following night he called so that she could wish her children good night. She accepted the call, but called police after he made inappropriate comments. The officers later told her they had called Lamontagne to remind him of his conditions.

Later that night, the coroner reported, Lamontagne asked a neighbour to borrow a gun because he said he was worried about his safety. The neighbour refused. It was the last time Lamontagne was seen alive.

Tristan and Antoine Lamontagne were found dead on a bed the next day, on Aug. 26, underneath a tent connected by a hose to a helium canister. Their deaths were ruled a homicide.

Ian Lamontagne was found dead on the premises, and his death was ruled a suicide.

Cantin says the province's Public Safety Department as well as the police training school should ensure that current and future police officers are given more training on how to recognize potentially dangerous situations within families. Enlisted officers should receive a refresher day on domestic violence, he added. 

As well, he suggested the Justice Department develop a protocol for mediators to end counselling sessions when they observe coercive control, and refer potential victims to aid services.

This report by ºÃÉ«tvwas first published June 17, 2025.