The British founder of the Syrian White Helmets civil volunteer group who was found dead in Turkey this week had received prolonged medical treatment for stress and depression, his family and friends have revealed.

Former British Army officer James Le Mesurier had visited a hospital the day before his body was found outside his home in Istanbul. He had been diagnosed with serious hypertension and doctors said his condition needed to be carefully monitored.

Turkish investigators told The Independent that they believe the former army officer committed suicide. They found no sign of violence towards Mr Le Mesurier either in a post-mortem investigation or during examination of the evidence collected, including CCTV footage.

Emma Winberg, Mr Le Mesurier’s widow, is believed to have told investigators that her husband had talked about taking his own life in the past. The police are in possession of his medical records.

Mr Le Mesurier’s body has been returned to England. His friends confirmed that Ms Winberg has been asked by the Turkish authorities to remain in the country while inquiries continue and said she has no objections in doing so.

The humanitarian, who received an OBE in 2016 for his services to Syrian civilians, was believed to be increasingly concerned about a reduction in funding for the White Helmets from international organisations and governments.

The White Helmets assist in Douma, Syria Show all 14 1 /14 The White Helmets assist in Douma, Syria The White Helmets assist in Douma, Syria Douma, the largest town in the Eastern Ghouta area with more than 100,000 residents, is surrounded and regularly shelled by regime forces EPA The White Helmets assist in Douma, Syria epaselect epa05635971 Volunteers of the Syrian Civil Defense, also known as the White Helmets, carry large pieces of burned wood as they try to extinguish a fire after a bombing in Douma, on the outskirts of Damascus, Syria, late 17 November 2016. According to local reports, at least six people were killed and 30 injured in several bombings mostly throughout the night. EPA/MOHAMMED BADRA EPA The White Helmets assist in Douma, Syria epaselect epa05636079 A volunteer of the Syrian Civil Defense, also known as the White Helmets, looks at a burning structure after a bombing in Douma, on the outskirts of Damascus, Syria, late 17 November 2016. According to local reports, at least six people were killed and 30 injured in several bombings mostly throughout the night. EPA/MOHAMMED BADRA EPA The White Helmets assist in Douma, Syria Members of the Syrian Civil Defense, known as the White Helmets, extinguish fire following a reported government airstrike on the rebel-held town of Douma, on the eastern outskirts of the capital Damascus, on November 17, 2016. Douma, the largest town in the Eastern Ghouta area with more than 100,000 residents, is surrounded and regularly shelled by regime forces. / AFP PHOTO / Sameer Al-DoumySAMEER AL-DOUMY/AFP/Getty Images Getty The White Helmets assist in Douma, Syria A Syrian man searches for victims as fire spreads following a reported government airstrike on the rebel-held town of Douma, on the eastern outskirts of the capital Damascus Getty The White Helmets assist in Douma, Syria Members of the Syrian Civil Defense, known as the White Helmets, extinguish fire following a reported government airstrike on the rebel-held town of Douma Getty The White Helmets assist in Douma, Syria Members of the Syrian Civil Defense, known as the White Helmets, extinguish a fire following a reported government airstrike on the rebel-held town of Douma, on the eastern outskirts of the capital Damascus Getty The White Helmets assist in Douma, Syria Syrians run for cover following a reported government airstrike on the rebel-held town of Douma, on the eastern outskirts of the capital Damascus Getty The White Helmets assist in Douma, Syria Members of the Syrian Civil Defense, known as the White Helmets, search for victims following a reported government airstrike on the rebel-held town of Douma, on the eastern outskirts of the capital Damascus, on November 17, 2016. Douma, the largest town in the Eastern Ghouta area with more than 100,000 residents, is surrounded and regularly shelled by regime forces. / AFP PHOTO / Sameer Al-DoumySAMEER AL-DOUMY/AFP/Getty Images Getty The White Helmets assist in Douma, Syria A Syrian man searches for victims following a reported government airstrike on the rebel-held town of Douma, on the eastern outskirts of the capital Damascus, on November 17, 2016. Douma, the largest town in the Eastern Ghouta area with more than 100,000 residents, is surrounded and regularly shelled by regime forces. / AFP PHOTO / Abd DoumanyABD DOUMANY/AFP/Getty Images Getty The White Helmets assist in Douma, Syria A member of the Syrian Arab Red Crescent searches for victims following a reported government airstrike on the rebel-held town of Douma, on the eastern outskirts of the capital Damascus, on November 17, 2016. Douma, the largest town in the Eastern Ghouta area with more than 100,000 residents, is surrounded and regularly shelled by regime forces. / AFP PHOTO / Abd DoumanyABD DOUMANY/AFP/Getty Images Getty The White Helmets assist in Douma, Syria Injured Syrian men receive treatment at a makeshift hospital following a reported government airstrike on the rebel-held town of Douma, on the eastern outskirts of the capital Damascus, on November 17, 2016. Douma, the largest town in the Eastern Ghouta area with more than 100,000 residents, is surrounded and regularly shelled by regime forces. / AFP PHOTO / Abd DoumanyABD DOUMANY/AFP/Getty Images Getty The White Helmets assist in Douma, Syria An injured Syrian man receives treatment at a makeshift hospital following a reported government airstrike on the rebel-held town of Douma Getty The White Helmets assist in Douma, Syria Injured Syrians receive treatment at a makeshift hospital following a reported government airstrike on the rebel-held town of Douma, on the eastern outskirts of the capital Damascus, on November 17, 2016. EPA

Some of the money was spent on support for volunteers who have been injured, and for bereaved families of those killed. A number of countries including the UK, US, France, Denmark and the Netherlands have contributed to the White Helmets. London and Washington DC have maintained their funding, but the situation regarding other states remains unclear.

Mr Le Mesurier had also been increasingly affected, say those close to him, by relentless attacks on social media. The Russian state has targeted him, as have and supporters of the Assad regime, falsely accusing the White Helmets of being linked to terrorist groups including al-Qaeda.

Ben Nimmo, an analyst with social media investigator company Graphika, said: “The White Helmets in general, and James Le Mesurier in particular, were subjected to the most sustained state-backed smear campaign that I remember seeing. It accused them of being ‘propaganda’, and it looked like it was because what they did was provide evidence of probable war crimes in Syria.

“The campaign began even before Russia launched its bombing operations in Syria in September 2015. First, pro-Assad bloggers and conspiracy theorists began claiming that the White Helmets were a propaganda outlet or a branch of al-Qaeda.”

He added: “The Kremlin’s outlets amplified those claims. Then the pro-Assad bloggers amplified the Kremlin. It was a barrage of tag-team trolling.”

And this harassment campaign only intensified, he said.

“The campaign really picked up in the second half of 2016, when Syrian and Russian forces launched the siege of Aleppo. Open-source evidence showed that the besieging forces were targeting hospitals and launching indiscriminate attacks on built-up areas.

“Those were potentially war crimes, and the White Helmets were the main witnesses. The response by the Syrian and Russian governments and their allies was to try and silence the witnesses – on the ground with airstrikes, and on the internet with the smear campaign.”

Mr Le Mesurier’s death came a week after Maria Zakharova, Russia’s foreign ministry spokesperson, accused him of being a British agent who had “been spotted all around the world, including the Balkans and the Middle East.”

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There have been repeated allegations, without any evidence produced, that Mr Le Mesurier was murdered. The Turkish authorities are said to be concerned that the conspiracy theories are interfering with their investigation and have offered his family the imposition of a news blackout. It remains unclear how exactly this would be enforced.

Karen Pierce, the UK ambassador to the United Nations, stated that claims that Mr Le Mesurier was a spy were “categorically untrue”. The Independent understands that Mr Le Mesurier had never worked for any British security or intelligence agency.

After Sandhurst, where he graduated top of the class and won the Queen’s Medal, he served in the Royal Green Jackets where General Sir Nick Carter, the current head of the British military, was a fellow officer.

Bashar al-Assad, however, maintains that Mr Le Mesurier was assassinated by western intelligence services as part of a plan targeting people who knew too many secrets. Another recent example of this, claimed the Syria president, was the death of sex offender Jeffrey Epstein, who committed suicide in an American prison while awaiting trial.

He claims other victims of this elimination programme were Osama bin Laden and Isis leader Abu Bakr al-Baghdadi. They were not killed because they were terrorist leaders responsible for thousands of deaths including American citizens, insisted Mr Assad, but “chiefly because they knew major secrets”.