09/11

On 11 September 2001 four passenger aircraft were hijacked in the skies above the eastern United States. Two of the planes were flown into the twin towers of the World Trade Center in New York City, causing both to collapse; a third was flown into the Pentagon - the headquarters of the US military; the fourth crashed in a field in Pennsylvania, following resistance from passengers onboard. In all about 3,000 people were killed, including about 300 firefighters and other emergency workers at the World Trade Center. 

(from : https://www.bbc.co.uk/teach/school-radio/articles/zsg8kty)