On Wednesday, it was announced that ten members of staff from The Halo Trust had been killed by an armed group at a mine clearance camp in Afghanistan.
The charity works toward clearing landmines in war zones, and had a close connection with Princess Diana.
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The charity also meant a lot to her son, Prince Harry, who returned to a landmine site visited by his mother 22 years after she first did to continue raising awareness.
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In the wake of the attack, the Duke of Sussex released a moving tribute on The Halo Trust's website, commemorating the people who lost their lives.
In the statement, he said: "In all, 26 members of The HALO Trust's Afghanistan team were killed or injured on Tuesday night in what was nothing less than an act of barbarism. I honour those who have been lost and encourage support for the survivors and the families of those affected.
"Those who work for HALO in Afghanistan face risks every day to remove the lasting—and still deadly—scars of war and conflict. The men who were attacked come from the very communities in which they work. They joined HALO to protect and restore their country and their homes. As I understand it, the deminers who lost their lives were also protecting their friends.
The royal commemorated those who were killed
"These workers put their lives on the line every day to make the world a safer place. This brutal act reminds us that we must stand in solidarity with humanitarian aid workers and the communities they serve."
He finished by saying: "I would urge all of HALO's supporters across the world to rally to their cause and help in any way they can."
Princess Diana famously walked through one of Halo's minefields in Angola in 1997; shortly after her visit, the Ottawa Mine Ban Treaty was signed, calling for all countries to rid the world of landmines.
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The news came ahead of Prince Harry's planned reunion with his brother Prince William in which the pair will commemorate their late mother's 60th birthday.
Princess Diana had a close connection with the charity
They will both attend the unveiling of a statue of Diana at Kensington Palace on 1 July, which is being installed in the Sunken Garden.
Kensington Palace previously stated: "The Princes hope that the statue will help all those who visit Kensington Palace to reflect on their mother's life and her legacy."
Back in 2017, it was announced that William and Harry had chosen renowned artist Ian Rank-Broadley for the piece, known for his portrait of the Queen on British coins.
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