Witnesses say the attacker did not say anything as he ran through a Pennsylvania high school Wednesday, slashing fellow students with two knives, reports CBS News.
The attack at Franklin Regional High School in Murrysville, Pa., left 21 students and a security guard injured. At least three attack victims remain hospitalized.
The school is closed Thursday as officials investigate what motivated the attack.
Shackled and dressed in a hospital gown, Alex Hribal, 16, was charged as an adult Wednesday with four counts of attempted homicide and 21 counts of aggravated assault.