The suspected attacker, who was not identified, was in police custody, Dan Stevens, deputy emergency management coordinator for the Westmoreland County, told reporters.
The attacks began around 7:13 am EDT in several classrooms and hallways as the classes were starting at Franklin Regional High School in Murrysville, 20 miles east of Pittsburgh, officials said.
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"What we teach is, the first thing you want the students to do is run," Seefeld said. "The fire alarm being pulled probably assisted with evacuating the school."
Seven students between the ages of 15 and 17 and one 60-year-old adult were admitted to Forbes Regional Hospital in Monroeville, Pennsylvania, with stab wounds.
Three of the patients were in surgery and all eight were listed in critical condition, hospital spokesman Jesse Miller said in a telephone interview.
All three undergoing surgery had been stabbed in the chest and abdomen, said Chris Kauffman, director of trauma at Forbes Regional Hospital.
"Patients who are stabbed in the abdomen and chest by definition have life-threatening injuries," Kauffman said.
A ninth patient who had arrived at the hospital was air-lifted to another facility, Miller said.
A 15-year-old girl who had been transported to another area hospital, Allegheny General, was discharged after being treated for "superficial" stab wounds, a hospital spokeswoman said.
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