Turkish investigators say they have so far found no evidence of foul play in the death of the former British military officer James Le Mesurier, who helped to establish Syria’s White Helmets rescue group, as his body was prepared for repatriation to the UK.

Police have reviewed camera footage and searched the apartment near where Le Mesurier, 48, was found with fractures to his legs and head on Monday morning. Forensic investigators carried out an autopsy on Tuesday and his remains were taken to Istanbul airport to be sent to London, Turkish state media reported.

Le Mesurier’s body was scheduled to be returned aboard a Turkish Airlines flight on Wednesday evening, the Anadolu news agency said.

There has been speculation that the death of the Briton, who along with the White Helmets had been the subject of an intense, Kremlin-sponsored disinformation campaign, may have come about by suspicious means.

A Turkish police official told the Guardian on Wednesday they had not yet found any such evidence. “From the first day investigators though this to be a suicide,” the official said. “Nothing in the forensic evidence has changed that view.”

Turkish media reports claimed a preliminary autopsy had found no evidence of a struggle or the use of a weapon. A final report is expected to be delivered within a week.

Local news quoted Le Mesurier’s wife, Emma Winberg, as telling police that he had given her a sleeping pill at about 2.30am on Monday, but she was woken up a short time later by police who said they had found his body on the street outside their apartment block.

Friends have said Le Mesurier, who helped to establish and train the White Helmets in 2014, was under pressure as a result of the coordinated attacks on his character and the stress of running a high-profile NGO.

The White Helmets, officially known as Syrian Civil Defence, operate in opposition-held areas to free civilians trapped in the aftermath of bombings. They have also documented several alleged war crimes, including the use of chemical weapons.

Their work has earned them Nobel peace prize nominations and a documentary on the group won an Oscar in 2017. But they have also been targeted by the Syrian government and its Russian allies, who have labelled the group a terrorist organisation and alleged, without presenting evidence, that Le Mesurier worked for western intelligence.

Many of the White Helmet’s more than 3,000 volunteers have since been resettled in other countries but hundreds remain on the ground in Idlib, the last rebel-held province in Syria.

Karen Pierce, the UK’s representative to the United Nations, said earlier that the British government would be “very closely” monitoring the investigation into Le Mesurier’s death. “I hope the Turkish authorities will be able to investigate thoroughly, and I’m sure we’ll want to give them any assistance they might require,” she said.

Claims that he worked for British secret services were “categorically untrue”, she said.