Police are at a loss to explain what prompted a 16-year-old boy to flail away with two kitchen knives, stabbing and slashing 21 students and a security guard in the halls of his high school in the US state of Pennsylvania.
The attack by Alex Hribal 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 on 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."
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At a brief hearing on Wednesday night, District Attorney John Peck said that after he was taken into custody, Hribal made comments suggesting he wanted to die.
Defence lawyer Patrick Thomassey described him as a good student who got along with others, and asked for a psychiatric examination.
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 millimetres, doctors said.
The rampage comes after decades in which US schools have focused their emergency preparedness on mass shootings, not stabbings.
There have been at least two major stabbing attacks at US 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.
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.
Hribal 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, with students yelling for everyone to run.
Assistant principal Sam King heard the commotion and found a chaotic scene in the blood-soaked hall.
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.
"There are a number of heroes in this day. Many of them are students," Governor Tom Corbett said in a visit to the stricken town. "Students who stayed with their friends and didn't leave their friends."
He also commended cafeteria workers, teachers and teacher's aides who put themselves at risk to help others.
Looking for a motive, Murrysville Police Chief Thomas Seefeld said investigators were checking reports of a threatening phone call between Hribal and another student the night before. He didn't say whether the suspect received or made the call.
