James Le Mesurier, a former British army officer, who had founded the organisation that trained the Syrian rescue group known as the White Helmets, was found dead in Istanbul on Monday (local time).
The body of Le Mesurier was discovered near his home in central Istanbul's Beyoglu district by worshippers on their way to a mosque, the state-run Anadolu news agency reported.
The Istanbul governor's office said that "comprehensive administrative and judicial investigations into Le Mesurier's death have been initiated".
Local media reports said that his body was found with fractures to his legs and head outside of his apartment.
Meanwhile, the police were further quoted as saying that no one had entered or left his home around that time and instead believed that he died after falling the balcony of his home office.
Authorities are investigating whether he committed suicide. Media reports have also alleged that Le Mesurier had been on anti-depressant pills.
Le Mesurier's wife told police that her husband had been taking medicine to treat "intense stress", Anadolu reported. His body was awaiting an autopsy.
Le Mesurier's Mayday Rescue group founded and trained the White Helmets, also known as the Syria Civil Defence.
In a statement on Monday, Mayday called him a "great leader and a visionary".
"James dedicated his life to helping civilians respond to emergencies in conflicts and natural disasters. Nowhere was the impact of his important work felt so strongly as in Syria," the international non-profit organisation said.
Le Mesurier was awarded an Order of the British Empire by Queen Elizabeth in 2016 for services to Syria Civil Defence and the protection of civilians in Syria.
"We have learned with shock and sadness the news of the death of @lemesurierjames, founder and director of the humanitarian organization @MaydayRescue, early on Monday at his home in Tophane in Istanbul, Turkey. Mayday is one of the institutions supporting the White Helmets," the group tweeted.
"The Syrian Civil Defence family extends its deepest condolences to the James family, and we express our deepest sorrow and solidarity with his family. As we also must commend his humanitarian efforts which Syrians will always remember," it added.
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