Alex Hribal did not have a history of mental illness, defense attorney Patrick Thomassey said, adding that he's not aware that the slender, dark-haired boy had been bullied, either.
He said the attack seemingly came out of nowhere.
"He's not a loner. He wouldn't be described by other students as a weirdo or anything like that. He's well liked. He's a B student. He comes from a great family," Thomassey told WTAE-TV, comparing his parents to those in the 1950s and '60s TV show "The Adventures of Ozzie and Harriet."
Hribal was charged Wednesday night with four counts of attempted homicide and 21 counts of aggravated assault. He was jailed without bail. Authorities said he would be prosecuted as an adult.
Authorities said Hribal, wielding two kitchen knives, stabbed and slashed students Wednesday morning as he ran down a hallway at Franklin Regional High School, about 15 miles east of Pittsburgh. The rampage began shortly after 7 a.m. and lasted about five minutes, ending when an assistant principal tackled the suspect and subdued him.
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. At the hearing, Thomassey described him as a good student who got along with others, and asked for a psychiatric examination.
The attorney 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.
Although some students have speculated that bullying might have prompted the boy's actions, Murrysville police Chief Thomas Seefeld has said he knows of no evidence to support the idea at this point.
Thomassey told several media outlets that 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 told ABC. "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 injured in the attack that left floors and walls stained with blood.
Police and doctors at a Pittsburgh hospital said one of students had surgery again overnight and is in "very critical" condition. Authorities haven't released the 17-year-old student's name.
Doctors at UPMC Presbyterian said he suffered a stab wound from a large knife that pierced his liver and missed his heart and aorta by millimeters.
Another injured student, Brett Hurt, 16, said Thursday he was terrified he might die when he realized he had been stabbed in the back.
"What was going through my mind?" said Hurt, appearing at a hospital news conference. "Will I survive or will I die."
Hurt said he doesn't think he could return to school anytime soon. "I might freeze," he said.
Officials said the high school will likely reopen Monday, after the school district hires a company to "clean and restore our building to pre-incident condition," Superintendent Gennaro Piraino said.
Associated Press writers Michael Rubinkam in northeastern Pennsylvania, Joe Mandak in Pittsburgh and JoAnn Loviglio in Philadelphia contributed to this report.