The British founder of a group that helped establish the Syrian White Helmets rescue organisation has died in Istanbul, his office said on Monday.
The body of James Le Mesurier was found with fractures to his legs and head outside the apartment building where he lived early on Monday, police said in a statement.
Le Mesurier was a former British Army officer who founded Mayday Rescue.
It helped train the White Helmets, a voluntary search-and-rescue group formed to respond to bombings by Syrian government forces in opposition-controlled parts of the country.
Police said Le Mesurier had recently required medical attention due to stress, and was using sleeping pills and antidepressants.
He returned late on Sunday night and went to bed. His wife was woken by police early in the morning to find her husband dead outside.
The Istanbul governor's office confirmed an investigation had been launched into the death.
The White Helmets, officially known as Syria Civil Defence, expressed their "shock and sadness" on Twitter.
Le Mesurier told Al Jazeera in 2015 that he had begun training and supporting the organisation in early 2013 alongside Turkish rescue experts, starting with "a single team of 20 people".
"I was working in Istanbul... and got together with a group of Turkish earthquake rescue volunteers," he said.
The White Helmets quickly expanded, and are credited with saving tens of thousands of lives during Syria's conflict.
A documentary about the group won an Academy Award in 2017.
The White Helmets have become a favourite target of pro-Syrian and pro-Russian groups. They have accused the group of supporting terrorists in Syria and doctoring footage of atrocities committed by regime forces -- claims strongly denied by its supporters.
Three days ago, the Russian foreign ministry tweeted about Le Mesurier, describing him as "a former agent of Britain's MI6, who has been spotted all around the world, including in the #Balkans and the #MiddleEast.
"His connections to terrorist groups were reported back during his mission in #Kosovo." the Russian ministry said.
Britain's ambassador to the United Nations, Karen Pierce, described Le Mesurier as "a true hero" and rejected the Russian accusations.
"He was a British soldier," she told reporters. "But... he wasn't a serving soldier when he founded Mayday and the White Helmets, he was a real humanitarian. And the world, and Syria in particular, is poorer for his loss."