The suspect in the Franklin Regional High School stabbing spree used two knives to attack at least 19 students and one security guard for five minutes before he was tackled by an assistant principal, police said.
The 16-year-old sophomore's "kitchen-type" knives were eight to 10 inches long. It was unclear how he got them into the Murrysville, Pa., school building.
"We were not aware of this individual" before the violent spree, Murrysville Police Chief Thomas Seefeld said at a news conference.
The teen's name was not disclosed.
Westmoreland County District Attorney John Peck said charges of aggravated assault would be filed shortly. He said the teen would likely be charged as an adult.
The suspect's computer was confiscated, but he did not have a cellphone, said Special Agent in Charge Scott Smith of the FBI’s Pittsburgh office, which was assisting in the investigation.
"Warrants are being executed," Smith said.
Many questions remained unanswered.
“We have really just scratched the surface about why this crime occurred, how this crime occurred," Peck said.
But authorities said it was clear that the toll could have been worse if the suspect was not subdued.
"Surely we averted a greater tragedy here today," said U.S. Attorney David Hickton.
An assistant principal, Sam King, tackled the suspect. The school resource officer, William "Buzz" Yakshe, a Murrysville police officer, took him into custody and he was in cuffs when other police arrived.
Seefeld described the scene that police encountered when they arrived at the school.
“We saw many victims in the grass area on the exterior of the building,” he said.
Inside, he said, “we came into a scene in one hallway where we saw one of the security guards had been apparently stabbed.
"It was a hallway that was pretty much in chaos, a lot of evidence of blood on the floor in the hallways. We had students running about,” he said.
Pennsylvania Gov. Tom Corbett commended the school community for its bravery and compassion in the face of horror.
"There are a number of heroes in this day. Many of them are students," he said.
Corbett said that while the motive for the attack was not apparent, the incident underscored the need for more attention to mental health.
“It’s premature to say what made this young man … what made him decide to get up today and do this?" the governor said. "What makes somebody do any violent act at any time?”
State Rep. Eli Evankovich said that the suspect's actions were "largely unpredictable" and he praised the response of students, staff and first responders.
“We’ll get through this. No doubt about that," he said.