(Adds details from police chief, possible charges, clarifies 20 students, 1 adult injured)
By Elizabeth Daley
MURRYSVILLE, Pa., April 9 (Reuters) - A 16-year-old student wielding two knives went on a stabbing rampage in the hallways of a Pittsburgh-area high school early on Wednesday, injuring 21 people before he was tackled by an assistant principal, officials said.
The attacker moved stealthily 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.
Students described a scene of panic, with the school hastily evacuated after a fire alarm was pulled. The unidentified sophomore suspected in the attack was in police custody, said Tom Seefeld, chief of police in Murrysville, Pennsylvania.
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" of about 8 to 10 inches (20-25 cm), and an armed security officer handcuffed him with help from the principal, Seefeld said.
"We had 20 students and one adult, a school safety officer, that were injured today," Westmoreland County District Attorney John Peck told a news conference,
He said the teen will likely be charged as an adult, possibly with aggravated assault and attempted murder, and then his name will be released.
Freshman Josh Frank said he did not initially realize that anyone had been stabbed, but fled when he heard screaming.
"He did it so stealthily that at first no one knew what was happening," Frank said. "We heard a girl scream bloody murder. Then two seniors were running down the hall and we followed them out of the school."
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.
"Apparently it was a large knife of some sort, because it was a large injury to his abdominal wall and went through his liver, diaphragm and major blood vessels," said Dr Louis Alarcon of the University of Pittsburgh Medical Center, who operated on a 17-year-old student. "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, most notably the December 2012 massacre of 26 children and adults at Sandy Hook Elementary School in Newtown, Connecticut, mass stabbings are less common.
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."
Police and the FBI were searching the 16-year-old 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."
A student at the school who witnessed the incident told CNN she had been in several classes with the suspected attacker, who was quiet and kept to himself.
As they were reunited with parents near the hilltop high school in the relatively affluent Pittsburgh suburb with a population of about 20,000, teens spoke about the incident.
Michael Float, an 18-year-old senior, described running down a staircase and finding a friend badly wounded.
"There was a pool of blood," Float said. "He had blood pouring down the right side of his stomach," and a teacher was applying pressure on the wound.
Zak Amsler, a 17-year-old junior, said the attack occurred just before his first class was about to begin.
"I saw a girl with blood running out of her sleeve," Amsler said as he waited to pick up his younger sister, a student at the nearby middle school. "It was pretty mind-blowing."
Pennsylvania Governor Tom Corbett said he had ordered state police to help local law enforcement respond to the incident. The FBI also said it had deployed agents to work with local law enforcement.
"As a parent and grandparent, I can think of nothing more distressing than senseless violence against children," Corbett said.
Gennaro Piraino, superintendent of the Franklin Regional School District, said the high school would be closed for the next two to three days while police conduct an investigation. (Additional reporting by Jonathan Allen in New York, Dave Warner in Philadelphia and Brendan O'Brien in Milwaukee; Writing by Scott Malone; Editing by Gunna Dickson)