(CNN) -- A male sophomore went on a stabbing spree at his high school in Murrysville, Pennsylvania, early Wednesday, the town's police chief said.
At least 20 people at Franklin Regional Senior High School, about 15 miles east of downtown Pittsburgh, were injured, and the alleged attacker is in custody, Thomas Seefeld said.
The violence happened in classrooms and a hallway, said Dan Stevens, a spokesman for Westmoreland County Emergency Management.
A school principal had "an interaction" with the suspect, and a school resource officer was able to handcuff him, Seefeld said. The 16-year-old is being treated for injuries to his hands, the chief said.
Mark Drear, vice president of the security company for the school, said one of his officers was stabbed. The school had three security officers and a full-time police officer Wednesday morning, he said.
A fire alarm that was pulled during the attack probably helped get more people out of the school during an evacuation order. Seefeld said that students were running everywhere, and there was "chaos and panic."
At one point, a female student applied pressure to the wounds of one of the male victims, possibly helping to save his life, said Dr. Mark Rubino, chief medical officer at Forbes Regional Hospital in nearby Monroeville, Pennsylvania, where seven teenagers and one adult were taken.
That male teen helped by a fellow student was one of three teens taken into surgery at Forbes.
The adult being treated there was not stabbed; he's suffering from an unspecified medical condition, according to hospital officials.
The teens' injuries are "quite serious," and "some are clearly life-threatening," said Dr. Chris Kaufmann of Forbes Regional.
They 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.
The teens who are undergoing surgery suffered knife wounds, most to the lower abdomen, Rubino said.
Physicians are evaluating other patients to see if they need surgery as well, Kaufmann said.
Rubino said he expects all the teens to live, noting that the strength of their youth gives them a greater chance of survival. But "I do want to stress the critical nature of their injuries," he cautioned.
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.
A doctor who treated six stabbing victims said most of them didn't immediately realize how they were injured. "They just felt pain and noticed they were bleeding. Almost all of them said they didn't see anyone coming at them," Dr. Timothy VanFleet, chief of emergency medicine at University of Pittsburgh Medical Center, told CNN's Ashleigh Banfield. "It apparently was a crowded hallway and they were going about their business, and then just felt pain and started bleeding."
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.
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."
Franklin Regional Senior High will be closed "over the next several days," district school Superintendent Gennaro Piraino said. The district's middle school and elementary schools will be open Thursday, and counseling will be available for the whole district, he said.
Information on what led to the stabbings and the conditions of the injured are still unfolding.
On 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.
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