The president and first lady were warmly welcomed when they arrived at the last of the 9/11 attack sites.
They went to Shanksville in Pennsylvania to lay a wreath at the memorial for those who died on Flight 93.
The first three jets had already hit their targets on that day in 2001 – and news made it through to the passengers on board the last hijacked airliner.
It is widely believed the terrorists who had taken over United Airlines 93 from Newark to San Fransisco had intended to target the White House.
But those on board had different ideas and tried to regain control of the aircraft.
It came down in a field near Shanksville on September 11th, 2001 killing the 40 passengers and crew on board.