At the sites where the United States came under attack from terrorists hijacking commercial aeroplanes, the bells toll for those killed in the deadliest attack on the country's soil since the World War Two bombing of Pearl Harbour.
At a ceremony at the 9/11 Memorial Plaza In New York, the names of the 2,983 killed are slowly read out by relatives...
"Patrick Sean Murphy."
"Raymond E Murphy."
"And my beloved father, Richard Barry Ross, and dearest friend Stacey Lee Sands."
Then, six minutes of silence.
Four to commemorate the exact times planes crashed into the World Trade Centre, the Pentagon and the field in Shanksville, Pennsylvania, where passengers thwarted a planned attack in the US Capitol in Washington.
The last two minutes of silence mark the dramatic moments when the North and South towers of the Trade Centre collapsed.
Dorna lost her brother in the attacks and flew in from London to attend the New York service.
"It's still hard. And coming here and seeing this for the first time since it happened, it's very hard, very hard but it's good that they've done... and it's a legacy for everybody, coming up with the kids of those who lost their parents and their children for them to see exactly what happened."
Marc Jones travelled from Boston to the memorial.
"I was a lot more emotional than I thought it was going to be. It was a very moving tribute and brought me right back to I remember exactly where I was when when I found out."
At a ceremony at the Pentagon in Washington, Barack Obama reflected on the tumultuous times since 9/11.
"Perhaps most of all, we stay true to the spirit of this day by defending, not only our country, but also our ideals."
The President noted how the fight against terror has changed in the intervening fifteen years.
"With our stronger defences, terrorists often attempt attacks on a smaller but still deadly scale. Hateful ideologies urge people in their own country to commit unspeakable violence. We've mourned the loss of innocents from Boston to San Bernardino to Orlando. Groups like al-Qaeda, like ISIL, know that they will never be able to defeat a nation as great and as strong as America so instead they try to terrorise in the hopes that they can stoke enough fear that we turn on each other and we change who we are and how we live."
Democrat presidential hopeful Hillary Clinton and her Republican opponent Donald Trump both issued statements honouring the victims.
Hillary Clinton was forced to leave the New York ceremony early after feeling faint.
But she later emerged in good spirits from her daughter Chelsea's home to face the waiting media.
"Madam Secretary how are you feeling?'
"I'm feeling great, great."
"What happened, what happened?"
On a day when campaigning in the fiercely-fought contest for the presidency was suspended to mark an atrocity that shook the world.
