MURRYSVILLE, Pa. (AP) — Doctors say one of the students wounded in a stabbing attack this morning at a Pennsylvania high school is on a ventilator, after a knife pierced his liver, missing his heart and aorta by just millimeters.

In all, at least five students were critically wounded and 15 other people were also hurt -- including a school police officer who confronted the assailant. The suspect is identified as a 16-year-old student. He's now in custody.

A witness says he had a "blank expression" on his face as he flailed away with two knives in the hallway just before the start of classes this morning at Franklin Regional High School, 15 miles east of Pittsburgh. Police say the motive is under investigation.

Doctors say they expect all the victims to survive, despite large and deep puncture wounds to the abdomen in some cases. The wounded campus police officer has been released.

Alyssa Finch, who said she is a student at Franklin Regional High School, told ABC News a fellow student pulled the fire alarm during the stabbing "because he knew what was happening ... and he wanted the people to get out."