A former British army officer who helped found the White Helmets volunteer organisation in Syria has been found dead in Istanbul, Turkish officials said.
James Le Mesurier's body was found near his home in the 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".
The Anadolu agency said police established that no one had entered or left his home at the time of the incident, and believe he may have fallen to his death.
Mr Le Mesurier was the founder and CEO of May Day Rescue, which founded and trained the White Helmets, also known as the Syria Civil Defence.
Anadolu also reported that Mr Le Mesurier's wife told police her husband had been taking medicine to treat "intense stress".
The White Helmets group confirmed his death on its Facebook page.
"The family of the Syrian Civil Defence extends its deepest condolences to his family, and we express our deepest sorrow and solidarity with his family, as it is our duty to commend his humanitarian efforts, which the Syrians will always remember," the group said.
The Syrian government and its allies, including Russia, have been critical of the White Helmets volunteers, accusing them of being agents of foreign powers, being terrorists for working in rebel-controlled areas and of staging chemical attacks.
The group, which has had more than 3,000 volunteers in opposition-held areas, says it has saved thousands of lives since 2013 and documented Syrian government attacks on civilians and other infrastructure.
Last week, Russian Foreign Ministry spokeswoman Maria Zakharova had accused Mr Le Mesurier of being a former British agent who has "been spotted all around the world, including in the Balkans and the Middle East".
"Given the role of the West in undermining stability in these regions, it is not difficult to assume what the British intelligence officer did there," she said.
Her comments were apparently prompted by a statement last week by a British Foreign Office official about the White Helmets coming under pressure.