Sophomore Alex Hribal was charged with attempted homicide and aggravated assault using 8-inch steel blades in a stabbing frenzy through the hallways of Franklin Regional High School in Murrysville near Pittsburgh on Wednesday.
A security guard and 21 students were wounded before the suspect was tackled by an assistant principal, officials and students said.
Three teenage boys remained hospitalized in critical condition with stab wounds, while other survivors and witnesses struggled on Thursday to explain the attack.
The teen had no psychiatric or disciplinary problems, and his family described him as a good student who mingled well with others, said family lawyer Patrick Thomassey in television interviews.
"He was not a loner," he told CNN on Thursday.
Thomassey said he met with Hribal for 20 minutes before his arraignment on Wednesday, but could not identify a motive.
"We're trying to figure that out," Thomassey told CNN. "This is a nice young boy. I mean, nobody would expect this. This is not a dysfunctional family. They're like the 'Brady Bunch.' His parents are active with their two sons."
Hribal's parents were horrified and sent their condolences to the victims and their families, he said.
The teen was charged as an adult, but Thomassey said he would try to move him into the juvenile court system.
Student athlete Brett Hurt, 16, described how he was goofing around with a friend before classes in the crowded hallway when suddenly it was filled with screams and, moments later, he was stabbed in the back, he said.
"That's just when everything went into straight chaos," he told reporters at the hospital on Thursday where he is being treated for his wound and a bruised lung. "What was going through my mind? Will I survive or will I die."
He said his friend helped him into a classroom and applied pressure to the wound to stanch the bleeding. Hurt, a weightlifter, said he had met Hribal only a couple of times, but harbored no ill will toward him.
"I've been thinking, maybe if he had more friends or somebody to help him out, to show him a different path. Maybe it would have been different," he said.
While the United States has seen numerous large-scale shootings in public places in recent years, including schools, movie theaters and shopping areas, mass stabbings are less common.
Police and the FBI on Wednesday searched the suspect's home. Neighbors said both parents work, and the teen has a brother who also attends Franklin Regional High School.
The high school will be closed, probably until Monday, while police conduct an investigation, officials said.
Murrysville Police Chief Tom Seefeld told CNN that authorities were investigating a report of a threat made the night before the stabbings, but had no evidence to support that.
Most of the victims were 14 to 17 years old.
Among the most seriously injured was a 17-year-old boy, who remained in critical condition following surgery at UPMC Presbyterian, a hospital spokeswoman said early on Thursday. A knife had passed through his liver, diaphragm and some major blood vessels, but missed his heart and aorta.
Eight other teenage boys were still recovering in two other hospitals on Thursday morning, including two who were in critical condition and may require more surgery.
Hribal faces four counts of attempted homicide and 21 counts of aggravated assault, police said. He was ordered to face a preliminary hearing in seven to 10 days.