In a packed Oshawa courtroom on Tuesday, 33-year-old Mark Gordon sat quietly inside the prisoner’s box as a judge delivered two concurrent life sentences with no eligibility for parole for 21 years following the fatal shootings of a Bowmanville school teacher and her ex-husband, a former Oshawa Generals hockey player.
Gordon formally admitted to his crimes in September 2025, pleading guilty to two counts of second-degree murder in connection with the July 13, 2024, killings of 42-year-old Patrick Montgomery and 42-year-old Andrieana Montgomery. The violent rampage, carried out in broad daylight along an Oshawa residential street, devastated a family and left the victims’ two young sons without parents.
The events of July 13, 2024, unfolded around noon when Gordon drove down Glenforest Street in Oshawa and pulled up to the house where Patrick Montgomery resided. Gordon stepped out of his vehicle, approached the residence, and knocked on the front door. Patrick’s 12-year-old son answered or looked out, recognizing Gordon as the tenant who rented the basement apartment at his mother’s nearby home.
Patrick walked outside to meet Gordon, and the two men stood in the driveway engaging in conversation for roughly 15 minutes. The peaceful interaction ended abruptly when Patrick’s 12-year-old son looked outside toward the driveway and saw Gordon pointing a handgun directly at his father. The young boy overheard his father utter a final plea—"please don’t"—seconds before Gordon opened fire, discharging multiple gunshots at point-blank range.
The tragic scene escalated in terror as Gordon prepared to leave the property. Statement of facts presented in court revealed that as Gordon turned away from Patrick’s body, he made eye contact with the 12-year-old boy. Overcome with fear and believing he was about to be executed next, the child instinctively raised his hands in the air.
Rather than turning the weapon on the boy, Gordon climbed back into his truck and drove approximately 400 metres down Glenforest Street to the property where his former neighbour, Andrieana Montgomery, rented the main upper level of a house. Andrieana, a dedicated teacher in Bowmanville and mother to two boys aged 10 and 12, was inside her home when Gordon arrived outside.
At the time of Gordon’s arrival, Andrieana was standing at her kitchen sink washing dishes while looking out through the front window of the residence. Gordon exited his truck armed with a handgun and immediately fired six shots from outside the building through the front structure. He then forced his way inside the home and fired an additional three shots at Andrieana. She was struck four times, including a fatal gunshot to the head.
The courtroom heard extensive background regarding the strained relationship that led to the fatal day. Gordon rented the basement suite of the Glenforest Street home where Andrieana lived upstairs. Gordon resided in the lower unit alongside his female partner and her three-year-old son, while Andrieana lived upstairs with her two sons.
The living arrangement was marked by ongoing friction. Court details established that Gordon and Andrieana did not maintain an amicable relationship and frequently engaged in disputes in the months preceding the shootings. The source of their persistent arguments primarily revolved around noise complaints emanating from both units.
The situation escalated significantly during the winter of 2023. Following a heated verbal dispute over noise, Gordon disclosed to Andrieana that he was in possession of a firearm.
Court documents confirmed that Gordon was indeed the lawful owner of firearms, holding a valid Possession and Acquisition License (PAL). His gun collection included a Glock 17 gen 5 semi-automatic 9 mm handgun as well as several rifles, all of which were routinely kept inside a metal safe located within his basement unit.
The revelation that her downstairs neighbour possessed firearms left Andrieana feeling deeply threatened. She lodged formal complaints with their shared landlord concerning the excessive noise coming from the basement unit and explicitly expressed her distress regarding Gordon’s gun ownership.
Fearing for her personal safety and that of her children, Andrieana repeatedly spoke to her 10-year-old and 12-year-old sons, extended family members, and close friends about the bitter ongoing conflict. She warned her children that Gordon was not a nice person, instructed them never to trust him, and admitted openly to those close to her that she was genuinely afraid of him.
In an effort to resolve the persistent problems with her living situation, Andrieana contacted municipal authorities on May 8, 2024. She filed an official report with the City of Oshawa stating that the basement apartment occupied by Gordon failed to comply with local legal requirements and building codes for a legal basement suite.
City of Oshawa officials acted on the complaint, contacting the property landlord regarding the code violations and outlining administrative options to address the illegal dwelling. Facing regulatory pressure, the landlord chose to proceed with evicting Gordon, his girlfriend, and her three-year-old child from the residence.
On July 12, 2024, Gordon was formally served with a notice of eviction. Less than 24 hours later, he retrieved his 9 mm handgun from his safe and launched his lethal attack on Patrick and Andrieana Montgomery.
Presiding over the sentencing, Madam Justice Michelle Fuerst delivered a stern assessment of Gordon’s conduct, characterizing the double murder as an act of extreme violence. Justice Fuerst stressed that Gordon carried out the homicides with full awareness that his actions would permanently deprive two young children of their parents.
"The callousness of his decision to act as he did in the face of that knowledge is immense," Justice Fuerst told the court.
Details from a presentence report offered additional insight into Gordon’s rationalization of his actions. In statements made to the report’s author, Gordon described feeling increasingly targeted as his relationship with his upstairs neighbour deteriorated. Gordon claimed he felt Andrieana was "bullying him" and stated that he "disliked being bullied." He explicitly identified the serving of the eviction notice as the ultimate trigger for the attack, describing it as the "last straw for him."
Gordon was arrested by police directly at the scene on July 13, 2024, and has remained in custody ever since. Addressing the court during his sentencing, Gordon offered a brief apology, stating that his actions were wrong, that he was the sole person responsible for the tragedy, and that he regretted everything he had done.
Justice Fuerst identified several severe aggravating factors in the case, including the fact that Gordon shot defenceless victims who posed no immediate threat to him. She emphasized the horrific cruelty of executing Patrick Montgomery directly in front of his 12-year-old child, leaving the boy traumatized in his own driveway.
The formal imposition of two concurrent life sentences with no eligibility for parole for 21 years closes the legal case against Gordon, ensuring he will serve more than two decades in federal custody before being eligible to apply for release.