The British founder of an organization that trains the Syrian White Helmets has been found dead in his home in Istanbul, according to reports. James Le Mesurier, founder of the Mayday Rescue group, was reportedly found dead early Monday. An unnamed diplomat told Reuters the circumstances around his death were unclear. The Mayday Rescue group is key in the training of the the White Helmets—known officially as Syria Civil Defense—who have been credited with saving hundreds of thousands of people in rebel-held areas during the country’s civil war. Le Mesurier and the White Helmets have both been targeted by Russian and Syrian authorities. Last week, the Russian foreign ministry accused Le Mesurier in a tweet of being a former British MI6 agent and having “connections to terrorist groups.”