An act of terror.

This is what an expert is calling America's deadliest massacre after a gunman shot into a music festival crowd.

The death toll stands at 59, with hundreds more injured.

US President Donald Trump said shooter Stephen Paddock was demented, but declined to call it domestic terrorism.

Terrorism studies Professor Bruce Hoffman told Andrew Dickens Trump's onto something because there's a difference between terror and terrorism.

"It can be an act of terror because terror is an emotion, for it to communicate a political message makes it terrorism. At the moment we just don't know the answers to any of those questions."

LISTEN ABOVE AS BRUCE HOFFMAN SPEAKS WITH ANDREW DICKENS