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MURRYSVILLE, Pennsylvania (CNN) -- High schooler Mia Meixner was tending to her locker at the start of her day, when a ruckus behind her got her attention. Police later identified the teen behind Wednesday's attack as 16-year-old Alex Hribal, and charged him with four counts of attempted homicide, 21 counts of aggravated assault and one count of possession of a weapon on school grounds.