MURRYSVILLE, Pa. — It was just before the start of class and the hallways were packed with students at their lockers or chatting with friends.

Nate Moore was walking to homeroom, book in hand, when a classmate he knew to be quiet and unassuming tackled a freshman boy a few feet in front of him. Moore thought it was the start of a fistfight and went to break it up.

But 16-year-old Alex Hribal wasn’t throwing punches — he was stabbing his victim in the belly, Moore said. The suspect got up and slashed Moore’s face, then took off down the hall, where authorities said he stabbed and slashed other students in an attack that injured 21 students and a security guard — and might have been even worse but for the “heroes” who Pennsylvania’s governor said helped prevent further injury or loss of life.

An assistant principal tackled and subdued Hribal, who was charged Wednesday night with four counts of attempted homicide and 21 counts of aggravated assault and jailed without bail. Authorities said he would be prosecuted as an adult.

The suspect’s motive remained a mystery.

“He wasn’t saying anything,” Moore recalled hours later. “He didn’t have any anger on his face. It was just a blank expression.”

At a brief hearing Wednesday night, District Attorney John Peck said that after he was taken into custody, 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.

Thomassey told ABC’s Good Morning America on Thursday that any defense he offers would likely be based on Hribal’s mental health. He said he hoped to move the charges against the teenager to juvenile court, where he could be rehabilitated. If convicted as an adult, Hribal faces likely decades in prison.

Thomassey said Hribal is remorseful, though he acknowledged his client didn’t appear to appreciate the gravity of his actions.

“At this point, he’s confused, scared and depressed. Over the next few days we’ll try to figure out what the heck happened here,” Thomassey said. “I think he understands what he did. … I don’t think he realizes how severely injured some of these people are.”

At least five students were critically wounded in the attack, including a boy who was on a ventilator after a knife pierced his liver, missing his heart and aorta by only millimeters, doctors said. He had additional surgery overnight, they said.

The rampage comes after decades in which U.S. schools have focused their emergency preparedness on mass shootings, not stabbings.

While knife attacks at schools are not unusual, they’re most often limited to a single victim, said Mo Canady, executive director of the National Association of School Resource Officers.

Nevertheless, there have been at least two major stabbing attacks at U.S. schools over the past year, the first at a community college in Texas last April that wounded at least 14 people, and another, also in Texas, that killed a 17-year-old student and injured three others at a high school last September.