Camilla Parker Bowles' brother, Mark Shand, has died at the age of 62.
Kate Middleton and Prince William sent their condolences from the South Pacific to the Duchess of Cornwall, who is married William's father Prince Charles.
"The Duke and Duchess of Cambridge were saddened to learn this morning of the tragic death of Mark Shand," the statement from the royal pair read.
"Their thoughts are with the Duchess and her family at this time."
Mark Shand died after suffering a head injury outside a New York nightclub on April 23. He slipped while lighting a cigarette and smashed his head on the sidewalk.
"The Duchess, The Prince of Wales, and all her family members are utterly devastated by this sudden and tragic loss," a statement from Clarence House read.
"Mark Shand was a man of extraordinary vitality, a tireless campaigner and conservationist whose incredible work remained his focus right up until his death."
The Elephant Family, his organization, helps prevent the extinction of endangered Asian elephants. Shand attended an auction for the charity on the day he died.
"Today we have lost the head of our family," the group said.
"Mark Shand was a true force for conservation. He was a legend and inspiration and above all, our great friend. We ask at this time that people's thoughts are with his loved ones."