Aine Davis, a former drug dealer in west London, fled to Syria in 2013 and joined up with ISIS.
He is one of the so-called “Beatles” – four ISIS members who have British accents.
It has now emerged that his grandfather was Rico Rodriguez, a Jamaican-born trombonist who played with The Specials, one of the biggest bands of the 80s.
He was awarded an MBE in 2007 for services to promoting reggae and ska music.
The Coventry-based ska band are famous for hit songs like Ghost Town and A Message To You Rudy. They broke up in 1984 but have re-formed twice and in 2013 did a big tour of the US to sold-out venues.
A source close to the family told The Sun: “This has been a horrible period for the family.”
In December it was reported that one of the Paris attackers had called Davis in Syria in the run up to the murderous assault in France.
He was arrested in Istanbul in November, only days before the attacks in Paris.
Davis, who has a firearms conviction in Britain, is married to Amal El-Wahabi.
Last year she was jailed in London for funding terrorism after she tried to trick a friend into carrying 20,000 euros (£14,000) to Davis in Turkey.
The trial of her glamorous friend, Nawal Msaad, made headline news because she had hidden the money in her knickers. Msaad was later acquitted.
Davis is known to have been close to Mohammed Emwazi – known as Jihadi John – who was killed in a drone strike last year.
Emwazi, a Kuwaiti-born Briton, appeared in videos of the beheadings of Western hostages.