MURRYSVILLE, Pennsylvania (CNN) -- A teenage boy wielding two kitchen knives went on a stabbing rampage at his high school in Murrysville, Pennsylvania, early Wednesday, before being tackled by an assistant principal, authorities said.
Twenty students and a security officer at Franklin Regional Senior High School were injured in the attack, District Attorney John Peck told reporters.
As authorities work to piece together a possible motive, the accused attacker -- a 16-year-old sophomore -- has been arraigned by a Pennsylvanian magistrate, said Dan Stevens, deputy emergency management coordinator for Westmoreland County.
The teen has been charged as an adult, a source close to the investigation told CNN. The source, speaking on condition of anonymity, said he has been charged with attempted homicide.
A doctor who treated six of the victims, primarily teens, said most initially did not know what happened.
"They just felt pain and noticed they were bleeding," Dr. Timothy VanFleet, chief of emergency medicine at the University of Pittsburgh Medical Center, told CNN.
"Almost all of them said they didn't see anyone coming at them. It apparently was a crowded hallway and they were going about their business, and then just felt pain and started bleeding."
'Don't know what I got going down'
The carnage began shortly before the start of classes, when an attacker began stabbing students in a crowded hallway and then went from classroom to classroom.
Student Matt DeCesare was outside the school when he heard a fire alarm ring and then saw two students come out of the school covered in blood.
Then he saw teachers running into the building and pulling "a couple of more students out," he told CNN. The students had been stabbed.
To stanch the bleeding, the teachers asked the students for their hoodies.
"We all took our hoodies off and handed them to the teachers to use as tourniquets to stop the bleeding," he said.
Recordings of emergency calls released in the wake of the attack provide a soundtrack of sorts to the terror and chaos that played out inside the school.
"I don't know what I got going down at school here but I need some units here ASAP," one officer can be heard saying.
Minutes later in another call, another official, breathlessly, can be heard detailing casualties: "About 14 patients right now."
Then another call for help. "Be advised inside the school we have multiple stab victims," one of the officers said. "So bring in EMS from wherever you can get them.
'Saw the kid who stabbing people'
Student Mia Meixner was standing at her locker.
"I heard a big commotion like behind my back," she told CNN's Wolf Blitzer. "And I turned around and I saw two kids on the ground."
She thought a fight had broken out, but then she saw blood.
"I saw the kid who was stabbing people get up and run away," she said.
Meixner said she saw three students help a bleeding freshman, saying they were taking him to a nurse. Then she saw a senior girl she knew.
"She was standing by the cafeteria doors. ... She was gushing blood down her arm."
Meixner dropped her books and went to help the girl.
"I started hearing a stampede of students coming down from the other end of the hall, saying 'Get out, we need to leave, go, there's a kid with a knife.' Then a teacher came over to me and the girl I was trying to help. And she said she would handle the girl and that I should run out. So then I just ran out of the school and tried to get out as soon as possible."
Blitzer asked whether the stabbing suspect said anything.
"No. He was very quiet. He just was kind of doing it," Meixner answered. "And he had this, like, look on his face that he was just crazy and he was just running around just stabbing whoever was in his way."
She said she didn't know the boy, but he had been in a lot of her classes. "He kept to himself a lot," she said. "He didn't have that many friends that I know of, but I also don't know of him getting bullied that much. I actually never heard of him getting bullied. He just was kind of shy and didn't talk to many people."
Tackled by an assistant principal
Assistant Principal Sam King is being credited with bringing the carnage to an end.
King tackled the teen, Peck told reporters. A school resource officer was able to handcuff the suspect, Police Chief Thomas Seefeld said.
The accused teen, who authorities have declined to immediately identify, was 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.
Stevens identified the police officer as William "Buzz" Yakshe, saying that he helped subdue the suspect. Yakshe is "doing fine," Stevens said. "He's more upset than anything else over what happened, because these are his kids."
A fire alarm that was pulled during the attack probably helped get more people out of the school during an evacuation order, Seefeld said. 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 was suffering from an unspecified medical condition, according to hospital officials.
Some injuries life-threatening
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.
Six victims were released from UPMC hospitals, VanFleet said, and two were in intensive care.
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.
'It doesn't happen here'
The attack in Murrysville is the latest in a string of school violence that has occurred across the nation. But mass stabbings, such as the one at the high school, are rare.
The attack has rattled the town, an upper-middle-class enclave with a population of about 20,000.
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 said he kept thinking, "It doesn't happen here, it can't happen here."
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.
CNN's Jason Hanna, Steve Almasy, Allison Malloy, Stephanie Gallman, Leigh Remizowski, Debra Goldschmidt and Shimon Prokupecz contributed to this report.