MURRYSVILLE, Pennsylvania — 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, about half of them seriously, officials said.

The attacker moved stealthily through Franklin Regional High School, stabbing his victims in the torso and slashing arms and faces before anyone realized what was happening, 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 attacks began around 7:13 a.m. in several classrooms and hallways of the school in Murrysville, 20 miles east of Pittsburgh.

An armed security officer subdued and arrested the attacker with help from the principal, Seefeld said.

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.”

A total of 21 people, most of them 14 to 17 years old, were transported to area hospitals, four by medical helicopters. Several had life-threatening injuries, with nine in critical condition, hospital officials said.

“Patients who are stabbed in the abdomen and chest by definition have life-threatening injuries,” said Chris Kauffman, director of trauma at Forbes Regional Hospital, where some of the injured were treated.

Doctors said a female student helped save a male schoolmate by applying pressure to his wound until emergency responders arrived.

The suspect was also being treated for injuries to his hands, Seefeld said.