New York will commemorate the 25th anniversary of the devastating 11 September 2001 attacks with a ceremony marked by US President Donald Trump's absence and right-wing outrage at the attendance of the city's first Muslim mayor.

Nineteen Al-Qaeda operatives hijacked four planes on that nearly cloudless day in 2001. Two jets slammed into the World Trade Center's Twin Towers, causing the iconic 110-story buildings to collapse.

Nineteen Al-Qaeda operatives hijacked four planes on that nearly cloudless day in 2001. Two jets slammed into the World Trade Center's Twin Towers, causing the iconic 110-story buildings to collapse.

President Donald Trump is set to observe the 25th anniversary of the Sept. 11 attacks by attending the Pentagon's ceremony Friday.