Defining images of the 9/11 attacks

Twenty years ago today, the US was changed forever by a terror attack that struck at the heart of the nation’s most populous city.

The deadliest terror incident on US soil claimed the lives of 2,977 people in all, after two planes were flown into the World Trade Center, a third plane hit the Pentagon in Washington DC and a fourth, on which passengers fought back against the hijackers, crashed into a field near Shanksville, Pennsylvania.

Here are some of the defining images of that day.