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 “comprehensive administrative and judicial investigations” had been initiated into Le Mesurier’s death.
Anadolu said police established that no one had entered or left his home at the time of the incident, and they believe he may have fallen to his death.
Le Mesurier was the founder and CEO of May Day Rescue, which founded and trained the White Helmets, also known as the Syria Civil Defense, a group of local humanitarian volunteers. Anadolu said he was 48 and had moved to Turkey with his wife four years ago.
The White Helmets confirmed his death on its Facebook page.
“The family of the Syrian Civil Defense 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 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.
Turkey’s private DHA news agency said authorities were investigating whether Le Mesurier had committed suicide, saying he had allegedly been on anti-depressant pills. Anadolu also reported that Le Mesurier’s wife told police her husband had been taking medicine to treat “extreme stress” and that they had moved to the neighborhood to be close to a health center.
Anadolu said his wife told police they had been up until 4 a.m., then took sleeping pills. She was woken by the doorbell less than two hours later and saw her husband’s body from the open window of their 3rd-floor apartment.
DHA said Le Mesurier’s body was awaiting an autopsy.