BAGHDAD — The British founder of a group that supported rescue efforts to save thousands of Syrian civilians was found dead in Istanbul on Monday.

The body of James Le Mesurier, 48, a former British army officer, was found near his apartment in central Istanbul in the early hours of the morning, according to Turkish media reports, which said he had suffered fractures to his head and legs after apparently falling from a balcony. Circumstances surrounding his death were unclear, and the Istanbul governor’s office said an investigation had been launched.

Le Mesurier had been a towering figure in an against-the-odds effort to save civilians from the Syrian and Russian governments’ barrel bombs and airstrikes. The organization that he founded, Mayday Rescue, has trained a civilian rescue team in Syria known as the White Helmets and overseen international government funding to it.

“It is with very heavy hearts that Mayday Rescue must confirm the death of James Le Mesurier,” the organization said in a statement. “James 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.”

Le Mesurier was awarded an Order of the British Empire by Queen Elizabeth II in 2016 for his work in Syria.

When bombs strike residential neighborhoods that lie outside Syrian President Bashar al-Assad’s control, it is the White Helmets who respond, pulling families from the rubble and rushing casualties to the hospital. The group has been credited with saving thousands of lives and has documented chemical weapons attacks that spurred U.S. military action against targets linked to the Syrian army.

That work had made the White Helmets, and Le Mesurier, a prime target for attacks by the Syrian and Russian governments and media, which placed them at the center of conspiracy theories and claimed they had terrorist links.

“The White Helmets’ co-founder, James Le Mesurier, is a former agent of Britain’s MI6, who has been spotted all around the world, including in the #Balkans and the #MiddleEast,” the Russian Foreign Ministry wrote on its Twitter account on Friday, claiming he had a connection to “terrorist groups.”

Although the White Helmets has been forced into a smaller and smaller swath of territory, increasingly controlled by militants, it says it remains a neutral civil defense force, focusing rescue efforts on the 3 million civilians caught between hard-liners and a government pushing to retake territory.

In an interview with The Washington Post earlier this year, Le Mesurier said the accusations had taken a heavy toll on the rescue workers whom his organization supported.

“What you need to understand is that these guys are the ones still fighting to save lives. They didn’t get into this job because they were political. They were bakers, engineers and just normal people,” he said. “But when it was a choice between fleeing or saving lives, this is what they chose: to stay, and to make a difference.”

Sarah Dadouch in Beirut contributed to this report.