Twenty years ago the world changed forever when 19 al-Qaeda terrorists hijacked four commercial aircraft and launched a series of co-ordinated attacks on the United States.

The images of planes flying into the twin towers in New York, the burning wreckage of aircraft in a field and at the Pentagon, and the devastation left as the towers collapsed, are burned into the memories of the entire world.

Today on Please Explain, North America correspondent Matthew Knott joins Nathanael Cooper to look at what has occurred in the twenty years since the September 11 attacks.