11th September 2001
2021 - 20th anniversary
On 11 September 2001 (known as 9/11), Islamist extremists hijacked four planes that were flying above the US. Two of them were flown into the twin towers of the World Trade Center in New York.
Another was crashed into the Pentagon, the top military building in the capital city, Washington DC. The fourth plane crashed into a field, 80 miles from Pittsburgh, Pennsylvania.
It was the biggest terrorist attack ever on America.