The Prince of Wales and his wife, Camilla, are "utterly devastated" by the death of her brother, who fell outside a hotel bar and suffered a head injury, British royal officials said.
Mark Shand, chairman of an elephant conservation charity, was in New York for a charity auction at Sotheby's.
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The New York Police Department said Shand had arrived at the Gramercy Park Hotel's Rose Bar just before 1 a.m. ET Wednesday accompanied by a relative. He went out to smoke a cigarette around 2:30 a.m. and fell backward as he tried to re-enter through a revolving door, police said.
He was taken to a hospital, where he was pronounced dead at 11:25 a.m., police said. The cause of his death will be determined by the medical examiner's office.
Shand, 62, was known for his work as a travel writer and conservationist. He was the author of several books including Elephant Tales and River Dog: A Journey Down the Brahmaputra.
British royal officials at Clarence House said in a statement that Camilla, Prince Charles and all of her family members are "utterly devastated by this sudden and tragic loss."
"Mark Shand was a man of extraordinary vitality, a tireless campaigner and conservationist whose incredible work through The Elephant Family and beyond remained his focus right up until his death," the statement said.
Sotheby's said it was "deeply saddened" by the news of Shand's death and it was honoured to have helped him raise money for his causes.
Police detectives were at the Manhattan hotel on Wednesday afternoon, taking a look at the locations of security cameras. A worker at a building across the street said detectives had asked if the building had any security video, which it didn't.
A call to the hotel's representatives was not immediately returned.