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GREENSBURG, Pa. (KDKA/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.

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Alex Hribal 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 just 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, but all survived and have since recovered.

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

Pat Thomassey, Hribal’s attorney, says he is disappointed his client was not able to plead guilty to being “mentally ill,” which would have committed him to a mental institution along with prison time.

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“It’s very difficult. I have a hard time with this case, for 3 1/2 years now because I sincerely believe that Alex is mentally ill, but we didn’t have much choice but to enter this plea. I mean, we didn’t want to put all these students and families through a trial,” Thomassey said. “He’s not getting the treatment that he needs, but he’s alright. I mean, he’s a nice young man to tell you the truth. He’s grown up. He was 16 when this happened. He’s 20 now and he’s dealing with it. He understands what he did, so he’s not making any excuses. He stood up there like a man this morning and said, ‘I’m guilty.’

“It’s been a long time for the victims waiting for today. I think they’re happy about the fact that now they can see a light at the end of the tunnel. I’ve been impressed in talking with them over the last few hours how significant the amount of trauma that was inflicted on them as a result of the injuries,” Washington Co. District Attorney John Peck said.

Peck said his office would like to see the judge sentence Hribal to 30 to 60 years behind bars, which was offered but rejected by Hribal as a potential plea agreement.

“That was felt to be an appropriate sentence based on the circumstance, the amount of harm caused,” Peck said.

“I believe Alex deserves a sentence. I don’t think 30 to 60 years is appropriate,” said Thomassey.

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(© Copyright 2017 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)