As word of the incident spread and children streamed out of the school, parents told FoxNews.com were shocked by the attack.
“Nobody’s ever prepared for something like that as a parent,” Matt Provenzo, who had dropped his son, Mark, off at Franklin Regional High School moments after the stabbings, told FoxNews.com. "As he got out of the car, you could see students exiting the building and that the fire alarm was pulled … There were a few students who were visibly upset."
Another parent, Tim Graham, told FoxNews.com he picked up his daughter, who was safe. But some of her friends were among those injured, he told FoxNews.com.
“She is with me and she was very close to the incident and a couple of her friends are stabbed," Graham said. "It could've very easily been my daughter."
Dan Stevens, a spokeswoman for Westmoreland County emergency management, said a male student was in custody following the incident inside the school, which occurred before classes began. A security guard at the school, in an affluent community about 15 miles east of Pittsburgh, notified authorities at 7:13 a.m.
The student's motive for the attack wasn't immediately clear, Stevens said.
Four of the victims were taken to area hospitals via helicopter. One victim was later identified as an adult, but no identities were immediately released.
The overwhelming majority of students evacuated in a "remarkably orderly fashion," said Provenzo, whose son was not injured.
"I saw one or two [students] who were visibly upset," Provenzo told FoxNews.com.
Dr. Mark Rubino said eight victims were taken to Forbes Regional Medical Center, where three remained in surgery. Two other victims were listed in critical condition, he told reporters during a midday press conference.
"Most of them were to the lower abdomen and they seemed to actually almost have a pattern," Rubino said of the stab wounds. "Most of them were to the right lower abdomen and to the right flank and that created some of the criticality of their wounds and the nature of their injuries."
Seven teens and one adult are listed in serious condition at Forbes Hospital, West Penn Allegheny Health System spokeswoman Jennifer Davis said, ranging in age from 15 to 60. A ninth victim, a 15-year-old girl, was listed in good condition, she said.
Reese Jackson, the hospital's president and CEO, told the Pittsburgh Post-Gazette that one of the victims may have saved the life of another.
“A surgeon came out and congratulated one of the victims by saying she had saved the person’s life by applying pressure to the person’s wound,” Jackson told the newspaper.
Graham said the attacker, a sophomore whom he did not identify, had been a victim of bullying.
"My point is this could’ve been avoided," Graham told FoxNews.com, adding that his daughter had been previously assaulted at the school. "That's my personal opinion.”
Roberta Cook, a Franklin Regional School Board director and member of the safety committee, said the local schools have well-practiced procedures in place for such events. Cook said she was awaiting a briefing on the incident, but had heard that no one had been killed.
"But I'm just glad it wasn't a shooter, because if it had been a shooter, there would have been fatalities," Cook, whose children attended the school, told FoxNews.com.
Murrysville Police Chief Thomas Seefeld told reporters a student or staff member at the school pulled a fire alarm when the attack began.
"The fire alarm being pulled probably assisted with evacuating the school," Seefeld said.
One witness told the Pittsburgh Post-Gazette that the suspect was tackled by assistant principal Sam King. Another witness told KDKA.com that the stabbings took place in the science wing of the school.
School officials did not immediately return messages seeking comment. Parents have been notified and were allowed to pick up their children at the school.
"A critical incident has occurred at the high school," a message on the school district's website read earlier Wednesday. "All elementary schools are cancelled, the middle school and high school students are secure. Additional information will be released as soon as possible. Please keep our campus clear of traffic."
Copyright 2016 Nexstar Broadcasting, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.