Four students are believed to have been seriously injured at a high school near Pittsburgh, where a student with a knife stabbed or slashed others - injuring as many as 20.
Westmoreland County emergency management spokesman Dan Stevens said not all of the 20 injured were cut by the knife, though most were.
Some suffered scrapes and cuts in the mayhem that ensued at Franklin Regional High School in Murrysville.
Mr Stevens said it does not appear that any students suffered life-threatening injuries. He said all the injured are between 14 and 17-years-old.
The suspect, a male student, was taken into custody and being questioned.
Mr Stevens said the suspect used a knife, though he did not say what kind or why he attacked the others.