OpenAI said it flagged the account of Jesse Van Rootselaar months before a deadly attack in British Columbia.
The system identified the activity in June last year as linked to the “furtherance of violent activities”.
The company reviewed whether to notify the Royal Canadian Mounted Police.
It decided the content did not show an imminent and credible threat at the time.
Its policy requires clear risk of serious physical harm before a referral.
Last week, the 18-year-old killed eight people in Tumbler Ridge and then died from a self-inflicted gunshot wound.
Police said the victims included a teaching assistant and five students aged 12 to 13.
Investigators reported earlier mental-health-related contacts with authorities.
After the attack, OpenAI contacted the RCMP and shared information about the account.
The company said it will continue to assist the investigation.
The motive for the shooting remains unknown.
The massacre is Canada’s deadliest mass killing since the 2020 Nova Scotia rampage.
