The motive for a horrific knife rampage at a Pennsylvania high school remains a mystery because the teenage suspect is not talking and many of the victims remain in hospital, police have said.

A boy with a “blank expression” stabbed and slashed 21 pupils and a security guard with two kitchen knives at his high school before being tackled by an assistant principal.

A 16-year-old student wielding two knives went on a stabbing rampage in the hallways of a Pittsburgh-area high school early yesterday, injuring 21 people, about half of them seriously, officials said.