Mr Le Mesurier was "very well thought of by his fellow junior officers", said a former officer who served with him at the Army Training Regiment in Winchester. One Syrian expert said he had faced "unimaginable pressure and targeting as a result".
The Istanbul governor's office said "comprehensive administrative and judicial investigations into Le Mesurier's death have been initiated".
But the Syrian government and its allies Russian Federation and Iran have accused the White Helmets of openly aiding terrorist organisations. "James dedicated his life to helping civilians respond to emergencies in conflicts and natural disasters".
Le Mesurier was awarded an Order of the British Empire by Queen Elizabeth II in 2016 for services to Syria Civil Defence and the protection of civilians in Syria. "He appeared to have fallen from a height".
A second Turkish source claimed his flat could only be accessed using fingerprint identification.
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Its first responders often risk death from "double tap" bombing raids by Syrian government forces, who launch secondary bombing attacks on the volunteers once they head into bombed buildings to search for survivors in the rubble.
Le Mesurier's wife said in her statement that her husband had recently started to take sleeping pills and psychiatric medications due to stress disorder and the incident took place early in the morning while she was sleeping. "She said she was later woken by knocking on the door, and discovered that her husband was lying on the street surrounded by police". The outlet reports that suicide is among the versions police are working on.
Anadolu Agency says James Le Mesurier's body was found near his home in the Turkish city's Beyoglu district by worshippers on their way to a mosque to pray.. "We express our deepest sorrow and solidarity".
"The world, and Syria in particular, is poorer for his loss", she said. "He faced unimaginable pressure and targeting as a result".
Le Mesurier, who had also worked for the United Nations, was considered a founder of the White Helmets.