A 16-year-old student wielding two knives went on a stabbing rampage in the hallways of a Pittsburgh-area high school on Wednesday, wounding 22 people before he was tackled by an assistant principal, officials said.
The attacker moved furtively through Franklin Regional High School halls, stabbing his victims in the torso and slashing their arms and faces, students and officials said. Some of the injured taken to nearby hospitals were in critical condition, doctors said.
Alex Hribal, a 16-year-old sophomore, was taken into custody, said captain Rob Liermann of the Murrysville Police Department.
Charged as an adult, Hribal faces four counts of attempted homicide and 21 counts of aggravated assault, Liermann said, and was ordered to face a preliminary hearing in seven to 10 days.
Students described a scene of panic, with the school hastily evacuated after a fire alarm was pulled.
"He did it so stealthily that at first no one knew what was happening," said freshman Josh Frank. "We heard a girl scream bloody murder. Then two seniors were running down the hall and we followed them out of the school."
The attacker, described by a classmate as a quiet person who kept to himself, started his rampage at around 7:13 a.m. EDT (1113 GMT), walking along the hallways to several classrooms at the school in Murrysville, 20 miles (32 km) east of Pittsburgh, officials said.
Assistant Principal Sam King tackled the boy, who was armed with two straight knives about 8 to 10 inches (20 to 25 cm) long, and an armed security officer handcuffed him with help from King, said Murrysville Police Chief Thomas Seefeld.
Twenty-one students and a security officer were stabbed in the incident, said Dan Stevens, a spokesman for Westmoreland County emergency management.
Two other students suffered minor injuries trying to get out of the school, Stevens added. He added the teenage suspect was not counted among the wounded.
The suspect was also being treated for injuries to his hands, Seefeld said. By late afternoon, he said, one or two of the victims were "still pretty critical."
Among those praised for heroics during the incident was Nate Scimio, the student who pulled the fire alarm and helped shield classmates, witnesses said.
"There's not enough words to describe how much of a hero he is," classmate Trinity McCool posted on Facebook.
The victims, most of them 14 to 17 years old, were transported to area hospitals, four by medical helicopters. Several had life-threatening injuries, hospital officials said.
Dr Louis Alarcon of the University of Pittsburgh Medical Center operated on a 17-year-old student and said he had "a large injury to his abdominal wall." The knife "went through his liver, diaphragm and major blood vessels," he said. "Fortunately for this young man, the knife missed his heart and his aorta."
While the United States has seen a number of large-scale school shootings in recent years, mass stabbings are less common.
Police and the FBI were searching the suspect's home, situated at the end of a quiet cul de sac. Neighbors said both parents work and the teen has a brother who also attends Franklin Regional High School.
"I don't know him really well, but he's always said 'hi'," said neighbor Lori Renda, 47, who said he played with her own children. "The family is so nice. Very, very nice."