It was just before the start of class and the hallways were packed as usual 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.

The attack in Murrysville unfolded shortly after 7am, a few minutes before the start of classes at 1200-student Franklin Regional High School, in an upper-middle-class area 24.14km east of Pittsburgh.

Mia Meixner, 16, said the freshman boy who was tackled tried to fight back, then, when his assailant got off him, stood up and lifted his shirt to reveal a midsection covered in blood.

“He had his shirt pulled up and he was screaming, `Help! Help!'” said another witness, Michael Float, 18.

He had his shirt pulled up and he was screaming, `Help! Help!’

“He had a stab wound right at the top right of his stomach, blood pouring down.”

As students rushed to the boy’s aid, the attacker slashed Moore before taking off around a bend.

“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,” said Moore, whose gashed right cheek required 11 stitches.

The boy ran down about 60 metres of hallway, slashing and stabbing other students with kitchen knives about 8 to 10 inches long, police said.

The assault touched off a “stampede of kids” yelling, “Run! Get out of here! Someone has a knife!” according to Meixner.

Assistant Principal Sam King heard the commotion and found a chaotic scene in the blood-soaked hall.

“I’ve been stabbed,” he heard a student say.

I never saw him with a particular group of friends

King then saw Hribal stab a security guard, who leaned against the wall, bleeding from his stomach, according to a police affidavit. King tackled Hribal and kept him on the floor until a school police officer handcuffed him.

The rampage lasted about five minutes.

Meixner and Moore called the attacker a shy and quiet boy who largely kept to himself, but they said he was not an outcast and they saw no indication before the attack that 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.”

During the attack, the boy had a “blank look”, she said. “He was just kind of looking like he always does, not smiling, not scowling or frowning.”