Mass stabbing at U.S. high school

An attacker armed with two knives has gone on a rampage at a high school in the U.S. state of Pennsylvania, stabbing 21 students and a security guard. A suspect, named as Alex Hribal, 16, was arrested at Franklin Regional High School in Murrysville near Pittsburgh, police said. He was later charged with four counts of attempted homicide and 21 counts of assault. The wounded, some with serious stab wounds, were as young as 14.
Background:
Pennsylvania teen charged after wounding 22 in stabbing spree at high school [Fox News]
Up to 22 people stabbed at Pennsylvania high school [BBC]
Four school stabbing victims in critical condition [CNN]