Consider a recent example. Last month in Tumbler Ridge, British Columbia, an 18-year-old killed her mother and half brother at home, then opened fire at a secondary school she had attended, killing five students and an educator. In the aftermath of the shooting, amid the expected evidence of the shooter’s despair, there emerged an alarming trail of online activity: On Roblox, a game platform, the shooter had created a game simulating a mass shooting; her TikTok account reportedly featured reposted videos of a mass shooter; she belonged to a gore forum where users can post uncensored videos of violence, which has been frequented by other mass killers; and she had visited the online profile of a 15-year-old girl who killed two people at the Abundant Life Christian School in Madison, Wis., in 2024.