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James Le Mesurier, key backer of Syria's White Helmets, dies in Turkey

Syrian President Bashar al-Assad and his backers describe the emergency response group as tools of Western propaganda and Islamist-led insurgents.

A key backer of the group that trained Syria's White Helmets emergency response group has died in Turkey, the organization confirmed Monday.

White Helmets tweeted that they have learned “with shock and sadness” about the death of James Le Mesurier at his home in Istanbul.

James Le Mesurier, co-founder of the White Helmets, was found dead in Istanbul. The White Helmets

“The Syria Civil Defense family extends its deepest condolences to the James family, and we express our deepest sorrow and solidarity with his family,” the group said in a tweet. “As we also must commend his humanitarian efforts which Syrians will always remember.”

The organization, whose network of volunteer rescue workers helps communities in war-ravaged Syria prepare for and recover from attacks, did not reveal how Le Mesurier died.

NBC News has reached out to Mayday Rescue for further comment on Le Mesurier’s death. Turkish police were not immediately available for comment.

The White Helmets, known officially as Syria Civil Defence, have been widely hailed for saving more than 115,000 lives in rebel-held areas during years of bombing by Syrian government and Russian forces.

White Helmets members say they are neutral, but Syrian President Bashar al-Assad and his backers describe them as tools of Western propaganda and Islamist-led insurgents.

Le Mesurier founded Mayday Rescue, a Netherlands-based nonprofit that helps establish community-based rescue and recovery teams, and is one of the institutions supporting the White Helmets.

In a statement Monday, the organization said Le Mesurier 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," it said in an emailed statement, adding that Le Mesurier should be remembered "as a great leader" and "a visionary."

Istanbul police refused to comment on Le Mesurier's death and referred NBC News to the governor's office, which said that a detailed judiciary, forensic and administrative investigation is underway.

In 2016, Le Mesurier was awarded a knighthood by Queen Elizabeth II for the protection of civilians in Syria.