King Charles and Queen Camilla laid flowers at the September 11 Memorial in New York, in a sombre moment during their state visit to the United States.
It is the first time that the royal couple have been to the site, which remembers the nearly 3,000 people who lost their lives in the attacks of 25 years ago.
They met families of those who died and talked to some of the first responders involved in the rescue attempt, with New York's Mayor Zohran Mamdani and former Mayor Michael Bloomberg among the dignitaries present.
The visit took place amid very tight security, with a heavy police presence, road closures, checkpoints and helicopters circling overhead.
In the New York sunshine, the King and Queen left a bouquet of white roses beside the deep pools that now commemorate those who lost their lives at this spot.
A handwritten note said: "We honour the memory for those who so tragically lost their lives on 11th September 2001.
"We stand in enduring solidarity with the American people and in the face of their profound loss. Charles R Camilla."
There is a museum on the site, which is now a permanent memorial, in an open space surrounded by the city's skyscrapers.
The King and Queen also had conversations with representatives of charities who have supported the families of victims and educational groups who are preserving the memory of what happened here in 2001.
The importance of the 9/11 attacks had been raised by King Charles in his speech to the US Congress on Tuesday.
He had called on US lawmakers to maintain support for Nato and had given the example of how Nato members had rallied to help the US in the wake of the 9/11 attacks.
"This atrocity was a defining moment for America and your pain and shock were felt around the whole world," said the King.
And he told Congress the "same unyielding resolve is needed for the defence of Ukraine".
But there was another diplomatic complication from President Donald Trump, who seemed to suggest that the King would have followed his approach on Iran - an issue that has caused friction between the UK and US governments.
"I think if he were doing that, if that were up to him, he would have probably helped us with Iran," said President Trump.
Buckingham Palace was approached for comment.
On this third day of the royal trip, the Queen paid a visit to the New York Public Library to highlight the importance of literacy.
Queen Camilla explained how books could open up the world: "The first Americans I knew and loved were the characters I met in my treasured children's novels: Little Women, What Katy Did, Charlotte's Web," she said.
Actress Sarah Jessica Parker and former Vogue editor Anna Wintour were also at the event.
The Queen has been a longstanding campaigner for literacy and reading, and she read Winnie-the-Pooh to a group of children.
She also brought her own literary figure to donate to New York Public Library.
Since 1987, the library has had on display the original stuffed toys from the 1920s that inspired the Winnie-the-Pooh stories.
But since the 1930s, the toy of Roo, the baby kangaroo, has been missing.
The Queen brought a replacement Roo on the flight from the UK, with the toy made by the same firm that made the originals.
The Queen has promoted literacy through her Queen's Reading Room project - and the charity's CEO, Vicki Perrin, who has also travelled to New York, warned on Wednesday that we are in a "reading crisis".
She highlighted the social benefits of reading, which she said had "transformative benefits on mental health, brain health and social health".
The Queen also toured a collection at the library spanning 250 years of shared American and British literary history.
King Charles headed to an education and healthy food initiative in Harlem called Harlem Grown.
He helped a group of children feed some chickens, and planted lavender and mustard greens.
There was also a glitzy reception for the King's Trust, attended by the King and Queen and celebrities including Lionel Richie, Stella McCartney, Donatella Versace and Martha Stewart.
Speaking at the event, Richie, an ambassador for the Trust, said the King's historic address to congress on Tuesday was "so brilliant":
"A friend, giving advice as to what we should be doing, a wise friend," he added.
The King made a brief speech himself, thanking Richie for "his incredibly kind words".
He also poked fun at his last few days in the US, telling those in attendance: "I hesitate to make yet another speech because I think you may have had enough."
In the evening, New York's iconic Empire State Building was illuminated in red, white, blue and gold in honour of the King and Queen's visit.
The nearby JPMorgan Chase Tower building was also lit up with a Union Jack.
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