Deepening the mystery of what set off the violence, attorney Patrick Thomassey said Alex Hribal had no history of mental illness or troublemaking, didn't abuse drugs and was no outcast at school, where the lawyer described him as a B or B-plus student.
"In a case like this, it's pretty obvious to me that there must be something inside this young man that nobody knew about," Thomassey told The Associated Press.
The local prosecutor, meanwhile, said Hribal remained an enigma.
"We have very little information about him," Westmoreland County District Attorney John Peck said, "except for the fact that he was a student, his age, and how he was as a student."
Authorities seized the family's computer as they searched for clues to Wednesday's rampage at Franklin Regional High, about 15 miles from Pittsburgh. Authorities said Hribal armed himself with two kitchen knives and stabbed 21 students and a security guard before an assistant principal tackled him.
The slender, dark-haired boy who looks younger than his years was jailed without bail on four counts of attempted homicide and 21 counts of aggravated assault. Authorities are prosecuting him as an adult, but Thomassey said he will try to have the case moved to juvenile court.
He said he plans to get his client examined by a psychiatrist before a preliminary hearing on April 30.
"I think his mental state now is unstable. I'm not sure that he recognizes the enormity, if that's the word, of what has occurred," Thomassey said. "And I think in his own mind he's trying to figure out what happened here, as we all are trying to figure out what the heck happened here."
The attack seemingly came out of nowhere, the attorney said.
"Both parents are good parents. They're parents who pay attention to their kids, who eat dinner with their kids every day, who understand their kids' friends, who, you know, care about who they hang out with," Thomassey said.
Ten boys remained hospitalized, three in critical condition.
Police and doctors said one victim, a 17-year-old whose name was not released, had surgery again overnight and was in very critical condition after suffering a knife thrust that pierced his liver and missed his heart and aorta by fractions of an inch.
Another student, Brett Hurt, 16, told of being stabbed in the back.
"What was going through my mind?" Hurt said at a hospital news conference. "Will I survive or will I die."
Hurt said he pushed his friend Gracie Evans out of the attacker's way, and then after he was stabbed, she stayed with him and put pressure on the wound.
"Gracie was screaming and asking me if I was all right," Hurt said.
Hurt's mother, Amanda Leonard, said of Evans: "I've hugged her and kissed her. I have told her thank you. There is nothing in the world I can do for that girl that can thank her enough for what she has done."
As for the assailant, Hurt said he hopes that someday "I can forgive him, and everyone else who got hurt can forgive him. First of all, he needs to forgive himself."
A day after the rampage, students pondered what comes next for their school, which could reopen Monday after the blood-spattered floors and walls are cleaned up.
"It will never be the same, but you want it to be as close to the same as possible," said Jacob Roberge, a junior.
Roberge said that while "people are definitely mad" at Hribal, "more so, people want him to get help."
Associated Press writers Jesse Washington in Murrysville and Michael Rubinkam in northeastern Pennsylvania contributed to this story.