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.

It was just before the start of class and the hallways were packed with students at their lockers or chatting with friends.

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.