Scotland Yard confirmed on Tuesday that they had arrested a 61-year-old man in London on suspicion of conspiracy to commit murder as part of the investigation into the death of Abdul Hadi Arwani on April 7.
The Times newspaper named the suspect as Burnell "Bernie" Mitchell, a convert to Islam who was director of the An Noor mosque in west London, where Arwani preached.
Mitchell's sister Liz was the lead singer of 1970s band Boney M, best know for hit singles "Rivers of Babylon", "Rasputin" and "Mary's Boy Child". They sold more than 200 million records worldwide.
The singer was born in Jamaica, but emigrated to Britain with her family in 1963.
Leslie Cooper, 36, described by the British press as a Jamaican businessman, was to appear in court on Thursday after being charged with the murder.
Police on Thursday said they had arrested a third suspect, a 53-year-old woman, who remained in custody along with the 61-year-old man.
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Arwani, 48, was found in his car with gunshot wounds to the chest in London on Tuesday.
The investigation is being handled by counter-terrorism police.
Arwani reportedly fled Syria as a teenager after surviving the 1982 Hama massacre, in which the current president's father Hafez al-Assad sent troops to brutally crush an Islamist-led uprising.
Arwani was a preacher at a mosque in west London between 2005 and 2011.
Police say they are "open-minded" about the motive for his killing.
Arwani's son Murhaf called his murder a "heinous" crime and appealed for anyone with information to come forward.
"My father was a very well-loved figure in west London," he said.
"He was the most peaceful man you could ever wish to meet. He did not care what your background, race or status was. He did not care if you were rich or poor. He just wanted to help people in need."