(NBC News) Police have finished their search for evidence inside the Colorado high school where an 18-year-old senior armed with a shotgun, machete and several Molotov cocktails shot a fellow student Friday before taking his own life.
Investigators say Karl Pierson had likely been planning his attack for a long time, and that things apparently escalated when he was kicked-off the Arapahoe High School debate team after a trip to a national tournament this summer.
Police believe Pierson was trying to hunt down librarian and debate team coach Tracy Murphy when he shot Claire Davis in the head at point-blank range.
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