(CNN) -- A male sophomore went on a stabbing spree at his Pittsburgh-area high school early Wednesday, an emergency management agency spokesman said.
At least 20 students at Franklin Regional Senior High School in Murrysville were injured, and the alleged attacker is in custody, said Dan Stevens, a spokesman for Westmoreland County Emergency Management.
The violence, Stevens said, happened in classrooms and a hallway. The suspect was handcuffed by a school resource officer, Murrysville Police Chief Thomas Seefeld said, and a school principal had "an interaction" with the suspect, leading to his apprehension.
A fire alarm that was pulled during the stabbings "probably assisted with evacuating the school," Seefeld said. He did not say who pulled the alarm.
Four people were flown to hospitals by helicopter, Stevens said.
Seven teens and one adult were being treated at nearby Forbes Regional Hospital in Monroeville, Pennsylvania, according to Dr. Chris Kaufmann. Their injuries are "quite serious," and "some are clearly life-threatening," he said. Victims were stabbed in their torso, abdomen, chest and back areas, and two people were sent to surgery immediately after arriving, he said. Those two patients had low blood pressure, he said.
Three of the seven stabbed teenagers taken to Forbes are still in the operating room, chief medical officer Dr. Mark Rubino said. All of the seven suffered knife wounds, most to lower abdomen, he said.
The rest were being evaluated to see whether surgery was necessary, Kaufmann said.
Rubino said he expects all seven stabbed teenagers who were taken there to survive, "but I do want to stress the critical nature of their injuries."
The adult was not stabbed but is being treated for a medical condition, according to hospital officials, who declined to specify the adult's condition.
Eleven victims were taken to four University of Pittsburgh Medical Center hospitals, UPMC spokesman Cindy McGrath said. One was sent to UPMC Presbyterian; four were taken to Children's Hospital; one was taken to UPMC Mercy; and five were taken to UPMC East. She did not have ages or conditions of the victims.
The students who were hurt range in age from 14 to 17, Stevens said. All of the injuries are stabbing-related, such as lacerations or punctures, and there were no guns involved he, said.
The alleged attacker, 16, is being treated for injuries to his hands, Seefeld said, adding that the security guard is among the injured.
A message on the Franklin Regional School District's website said all of its elementary schools were closed after the incident, and "the middle school and high school students are secure."
The Murrysville school is about 15 miles east of downtown Pittsburgh. Information on what led to the stabbings and the conditions of the injured are still unfolding.
Later Wednesday morning, students were being released to their parents, Stevens said. Shortly before 10 a.m. ET, CNN affiliate KDKA reported that some parents were beginning to be reunited with their children.
Bill Rehkopf, a KDKA radio host and Franklin Regional High School graduate, reported on air that he was shocked by the stabbings.
He kept thinking, "It doesn't happen here, it can't happen here," he said.
He said he was seeing parents showing up at the school and an increasing media presence. Parents appear to be calm, he said. Another KDKA reporter said she spoke with a parent who said she received a cell phone call from her daughter, who told her mother that "something bad" happened and that she needed to be picked up.
CNN first learned of the stabbings on Twitter.