(CNN) A high-profile supporter of the "White Helmets" group, officially known as Syria Civil Defense, has been found dead in Istanbul, according to Turkish authorities.

James Le Mesurier, the 48-year-old founder and director of the Mayday Rescue group and an ex-British army officer, died in the Kemankes Karamustafapasa neighborhood early on Monday, the Istanbul governorship said in a statement.

Mayday, a non-profit based out of Istanbul and Amsterdam, supports Syria Civil Defense . The group was established in 2014 and has received funding from the UN and a number of foreign governments, including the UK. According to Mayday, the group has "trained, equipped and managed networks of local volunteers who have provided life-saving support to hundreds of thousands of civilians trapped in conflicts."

The White Helmets started as a volunteer rescue operation, filling the void left by the official emergency services.

White Helmets Director Raed al-Saleh told CNN the group had been informed of Le Mesurier's death on Monday.

Le Mesurier pictured during training exercises in southern Turkey in 2015.

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