A 15-year-old boy is in custody in Colorado, accused of stabbing a 19-year-old woman to death—and police say he planned to keep killing. 

The suspect, whose name has not been released, was arrested early Sunday, hours after Makayla Grote—a racetrack employee and aspiring NASCAR driver, notes KDVR—was fatally stabbed at her apartment in Longmont, CBS Denver reports.

 Per court documents, the teen exchanged "troubling text messages" weeks ago with Grote's younger sister that she reported to police. He allegedly attacked Makayla with a hunting knife when he arrived at the apartment around 6:15pm and she opened the door. Police say her younger sister locked herself in a bedroom and the teen left after he was unable to get inside.

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