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Murrysville, Pa. • Flailing away with two kitchen knives, a 16-year-old boy with a "blank expression" stabbed and slashed 21 students and a security guard in the crowded halls of his suburban Pittsburgh high school Wednesday before an assistant principal tackled him.

At least five students were critically wounded, including a boy whose liver was pierced by a knife thrust that narrowly missed his heart and aorta, doctors said. Others also suffered deep abdominal puncture wounds.

The rampage set off a screaming stampede, left blood on the floor and walls, and brought teachers rushing to help the victims.

Police shed little light on the motive.

The suspect, Alex Hribal, was taken into custody and treated for a minor hand wound, then was brought into court in shackles and a hospital gown and charged with four counts of attempted homicide and 21 counts of aggravated assault. He was jailed without bail. Authorities said he would be prosecuted as an adult.

District Attorney John Peck said that after he was seized, Hribal made comments suggesting he wanted to die.

Defense attorney Patrick Thomassey described him as a good student who got along with others and asked for a psychiatric examination.

The attack unfolded in the morning just minutes before the start of classes at 1,200-student Franklin Regional High School in an upper-middle-class area 15 miles east of Pittsburgh. It was over in about five minutes, during which the boy ran wildly down about 200 feet of hallway, slashing away with knives 8 to 10 inches long, police said.

Nate Moore, 15, said he saw the boy tackle and knife a freshman. He said he going to try to break it up when the boy got up and slashed Moore's face, opening a wound that required 11 stitches.

"It was really fast. It felt like he hit me with a wet rag because I felt the blood splash on my face. It spurted up on my forehead," Moore said.

The attacker "had the same expression on his face that he has every day, which was the freakiest part," he said. "He wasn't saying anything. He didn't have any anger on his face. It was just a blank expression."

Assistant Principal Sam King finally tackled the boy and disarmed him.

As for what set off the attack, Murrysville Police Chief Thomas Seefeld said investigators were looking into reports of a threatening phone call between the suspect and another student the night before. Seefeld didn't specify whether the suspect received or made the call.

The FBI went to the boy's house, where authorities planned to confiscate and search his computer.

On Wednesday, Mia Meixner, 16, said the rampage touched off a "stampede of kids" yelling, "Run! Get out of here! Someone has a knife!"

The boy had a "blank look," she said. "He was just kind of looking like he always does, not smiling, not scowling or frowning."

Meixner and Moore called the attacker a shy boy who largely kept to himself, but they said he was not an outcast and they had no reason to think he might be violent.

"He was never mean to anyone, and I never saw people be mean to him," Meixner said. "I never saw him with a particular group of friends."