The commemoration to mark 20 years since the attacks began with a tolling bell and a moment of silence at the 9/11 memorial plaza in New York City. President Joe Biden, and former presidents Barack Obama and Bill Clinton were among the crowd who fell silent to mark the moment the first plane struck. In all there were six moments of silence, marking the times that each tower was struck and the times that each fell, as well as the moment when the Pentagon was attacked and the time that United 93 crashed in Pennsylvania
