On 11 September 2001, at 9.03am, the second of four hijacked planes hit the South Tower of the World Trade Center in New York.
The US government quickly concluded: America was under attack.
President George W Bush was sitting in front of seven-year-olds in a classroom in Florida.
With limited information, officials fought to regain control.
The events of the day led to two decades of conflict in Afghanistan and Iraq.
Watch 9/11: Inside the President’s War Room on BBC iPlayer.