Crowds gathered at Ground Zero in New York to mark five years since the attacks of 11 September 2001.
The bright morning sunshine was reminiscent of the autumn day when two hijacked planes flew into the twin towers of the World Trade Center, eventually causing them to collapse.
President Bush joined fire-fighters for a ceremony of remembrance at the Fort Pitt Firehouse in Manhattan's Lower East Side.
In the depths of Ground Zero, people left tributes in two reflecting pools on the sites of the two fallen towers.
Some of those who came to pay their respects struggled to keep their emotions under control.
Patricia Smith, whose mother, Moira, a police officer, died in the attacks, took part in the ceremonies at Ground Zero.
New York Mayor Michael Bloomberg, who was not in office at the time of the attacks, spoke briefly to the crowd, describing the day as a "heartbreaking anniversary".
Friends and family of those who died left messages on a memorial wall, viewed here by three New York fire-fighters.
Commemorations also took place at the sites of the other attacks: in Pennsylvania people visited a memorial to the passengers who fought back against the hijackers.
Later on, at the Pentagon, President Bush and First Lady Laura observed a moment of silence for the 184 victims.