Pennsylvania high school stabbing

Emergency crews gather in the parking lot of Franklin Regional High School in Murrysville, Pa., after at least 20 people were injured in a stabbing attack.   (Keith Srakocic / Associated Press)

A woman walks onto the campus of the Franklin Regional Schools, where at least 20 people were stabbed, allegedly by a male student, at Franklin Regional High School on Wednesday.  (Gene Puskar / Associated Press)

A pair of women walk from the Franklin Regional Schools campus, where at least 20 people were stabbed at Franklin Regional High School on Wednesday.  (Keith Srakocic / Associated Press)

A Salvation Army disaster services vehicle drives past a school bus onto the campus of Franklin Regional Schools, where at least 20 people were stabbed at Franklin Regional High School.  (Keith Srakocic / Associated Press)

Students walk from the campus of Franklin Regional High School in Murrysville, Pa., after a mass stabbing left at least 20 people injured.  (Gene Puskar / Associated Press)