The man - who has not been officially identified - has been named locally as Abdul Hadi Arwani, a Syrian preacher from the Masjid An-noor mosque in Acton.
Known for his opposition of Syrian President Bashar al-Assad, Abdul was considered an authority on Islamic marriage contracts and divorce law.
Police were called to Greenhill, at the junction with The Paddocks, on Tuesday morning by London Ambulance Service who had discovered the body of Mr Arwani with gunshot wounds in a dark-coloured Volkswagen Passat.
Detectives appealed for anyone who was in the area on Tuesday morning, or anyone who saw anything suspicious locally, to get in touch.
Police cordons and road closures remained in place on Wednesday morning (April 8).
A local mechanic, who did not want to be named, spoke to getwestlondon. He said: “The roads were blocked off close to our store, it was blocked off not far from us.
“[There was] lots of police activity and a tent was set up too.
“It's quite peculiar for this area, we don't seem to get stuff like this on this kind of scale.
"Surely there must have been some witnesses that could come forward, it was the middle of the day and I’m sure there are many nosy neighbours on that road.”
The identity of the victim was outed online via social media, with an outpouring of grief and support for the preacher.
One Twitter user wrote: “The Muslims in the UK have lost a great Imam and a person of knowledge. May Allah have mercy on you Sheikh Abdul Hadi Arwani.”
Abdul Hadi Arwani was a survivor of the Hama massacre in 1982, where at least 10,000 civilians were killed as a result of a Syrian Army operation.
According to the London College of Islamic Studies, he had studied at the Faculty of Shariah (Islamis Law) in Jordan, and was a teacher in various Islamic seminaries in Slough and London.
Detectives from the Metropolitan Police Homicide and Major Crime Command declined to speculate on reasons or motives for the murder.