Joe Biden and former Presidents Barack Obama and Bill Clinton reunited to mark the 20th anniversary of 9/11. The politicians were joined by their wives, First Lady Jill Biden and former first ladies Michelle Obama and Hillary Clinton, as they attended the annual 9/11 Commemoration Ceremony at the National 9/11 Memorial and Museum on Saturday.During the ceremony, six moments of silence were held, marking when each of the World Trade Center towers was struck and fell.