ANKARA, Turkey (AP) — Turkish officials were performing an autopsy and other procedures Tuesday as they tried to understand how…

ANKARA, Turkey (AP) — Turkish officials were performing an autopsy and other procedures Tuesday as they tried to understand how a former British officer who helped found the White Helmets volunteer aid group in Syria died.

James Le Mesurier’s body was found near his home in Istanbul early Monday by worshippers on their way to morning prayers. Turkish police believe he fell to his death from his home and are investigating the circumstances. Last week a top Russian official had claimed he was a spy, something Britain strongly denies.

The Istanbul chief prosecutor’s office said an autopsy and other procedures were underway at Istanbul’s Forensic Medicine Institute to determine “the exact cause” of his death. It also said police were still in the process of gathering security camera recordings near the scene and assessing them.

Earlier, Istanbul governor Ali Yerlikaya told reporters: “Our chief prosecutor’s office, our police are engaged in multifaceted efforts to shed light on the incident.”

Le Mesurier was the founder and CEO of May Day Rescue, which established and trained the White Helmets, also known as the Syria Civil Defense, a group of local humanitarian volunteers.

The group, which has had more than 3,000 volunteers in opposition-held areas, says it has saved thousands of lives since 2013 and documented Syrian government attacks on civilians and other infrastructure. The group has been nominated for the Nobel Peace Prize, but has not won.

Last week, Russian Foreign Ministry spokeswoman Maria Zakharova accused Le Mesurier of being a former British agent working in the Balkans and the Middle East. She alleged he had “been spotted all around the world, including in the Balkans and the Middle East.”

Karen Pierce, Britain’s ambassador to the United Nations, denied those allegations Monday, saying “the Russian charges against him, that came out of Foreign Ministry that he was a spy, are categorically untrue.”

She also said Britain would be “looking very closely” at the Turkish authorities’ investigation into Le Mesurier’s death.

Turkey’s state-run Anadolu Agency reported that he was 48 and had moved to Turkey with his wife four years ago.

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