An FBI agent enters the family home of a suspect after a series of knife attacks at at Franklin Regional High School in Murrysville, Pennsylvania April 9, 2014. [Photo/Agencies]

Heroes

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 said 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.