The founder of the White Helmets and director of its most prominent support organization were found dead in Istanbul on Monday, his office confirmed.
The cause of the death of British-born James Lemisore, a former intelligence officer, was not immediately known, while his office declined to comment.
According to information published in the press, he was found dead at dawn Monday at the bottom of the building where he lives, and found fractures in his legs and head.
The office of the governor of Istanbul confirmed the death of Lumizuri, citing an investigation to determine the causes.
Turkish security sources quoted his wife as saying that he had recently started taking anti-depressants and sleeping pills because he was suffering from stress.
Police came to their apartment at about 2.30pm after being informed that a body had been found in the building, security sources said.
The "White Helmets" on her Twitter account expressed "sadness and sorrow" for his death, pointing out that "his humanitarian efforts (...) will be remembered by the Syrians always."
Lumizore, a former British army officer, ran the non-profit organization Mayday Rescue, which coordinates donations to the White Helmets and has offices in Istanbul and the Netherlands.
The "Mayday Rescue" on Twitter confirmed the death of Lumisore, and called on the media to "not make speculation" about the circumstances of the death.
On October 22, US President Donald Trump approved $ 4.5 million in assistance to the White Helmets.
The Syrian regime and its Russian ally accuse the White Helmets, which include volunteer paramedics working in the bombed areas, of supporting opposition and fighting factions in the areas.
The Russian Foreign Ministry said on Twitter that Lumizore, "a former agent in the MI6 (British intelligence) has links with terrorist groups."
The White Helmets were awarded the 2017 Academy Award for Best Short Documentary.
