GREENSBURG, Pa. (AP) — A man has pleaded guilty to stabbing 20 fellow students and a security guard at his suburban Pittsburgh high school more three years ago, and likely faces decades in prison when he returns for sentencing.

Alex Hribal (RY'-buhl) pleaded guilty Tuesday to 21 counts each of attempted homicide and aggravated assault, plus a weapons charge.

Hribal, who turned 20 earlier this month, was 16 when he used two eight-inch kitchen knives to stab and slash his way through the hallways of Franklin Regional High School in Murrysville on April 9, 2014.

Four students were critically injured at the school about 15 miles east of Pittsburgh, including one who required a liver transplant. All survived and have since recovered.

Hribal said nothing in court, other than to reply "guilty" to each charge.

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