An American high school student has stabbed or slashed 22 people, mostly teenagers, in a bloody 30-minute rampage in the classrooms and hallways of a Pittsburgh high school in the state of Pennsylvania, officials said.
The attack at Franklin Regional High School in Murrysville, a suburb of Pittsburg, began at 7:13am (1213 GMT) on Wednesday as students began arriving for what should have been a routine day.
He did it so stealthily that at first no one knew what was happening ... We heard a girl scream bloody murder.
The attacker moved furtively through 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.
"Twenty students were injured, four of those students were flown from the scene," Dan Stevens, a spokesman for Westmoreland County emergency management, told the AFP news agency.
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, Reuters news agency reported, 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 am local time (11:13 GMT), walking along the hallways to several classrooms at the school in Murrysville, 32 kms east of Pittsburgh, officials said.
Assistant Principal Sam King tackled the boy, who was armed with two straight knives about 8 to 10 inches long, and an armed security officer handcuffed him with help from King, said Murrysville Police Chief Thomas Seefeld.
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.