Ceremonies are taking place in the United States on Tuesday to commemorate the 11th anniversary of the 9/11 attacks in 2001.
At Ground Zero in New York a moment of silence was held to mark the exact time each of the planes struck the twin towers of the World Trade Centre.
Relatives read the names of those who died.
In Washington, President Barack Obama laid a wreath at the Pentagon and Vice-President Joe Biden was expected to speak at Shanksville, Pennsylvania, where one of the jets crashed as passengers attempted to overpower the hijackers.
Mr Obama said those killed would never be forgotten, and that they had "helped us make the America we are today".
"The true legacy of 9/11 will not be one of fear or hate or division," he said. "It will be a safer world, a stronger nation, and a people more united than ever before."
An outdoor memorial was opened at Ground Zero in New York last year and has since been visited by almost 4.5 million people.
Tuesday dawned with a clear blue sky over Washington and New York, jogging memories of a similar September morning 11 years ago.
President Obama and First Lady Michelle Obama observed a moment of silence at the White House at 08.45am.
They then went to the Pentagon to attend a memorial ceremony where one of the jets crashed, and then Walter Reed Army Medical Center to visit wounded soldiers.
Dunedin-born soprano Marla Rodriguez was invited to sing The Star Spangled Banner at a dawn ceremony.