The remains of a Russian airliner are seen as military investigators inspect the crash site in al-Hasanah area in El Arish city, north Egypt, November 1, 2015. (Reuters)

Abu Osama al-Masri, an Egyptian cleric and the frontman of an ISIS-offshoot group, has been unmasked as the suspected mastermind behind the alleged downing of a Russian jet, according to a media report."Intelligence chiefs believe that the plane, which crashed last weekend killing 224 people, was brought down by a bomb planted by an offshoot of ISIS in the Sinai peninsula," The Sunday Times reported.The group's frontman, Masri, an Egyptian cleric, brokered a pact with the Islamic State of Iraq and Syria (ISIS) last year in Syria, the report said.British officials confirmed this weekend that Masri is a "person of interest" and that Britain would help Egypt or Russia in a "kill or capture" mission. Such a move could involve the deployment of the SAS to Egypt, it said.The man known as Masri claimed responsibility for the Russian plane crash in an audio statement last on Wednesday --- the same day that David Cameron announced the suspension of British holiday flights to Sharm el-Sheikh.Intelligence officials believe that Masri's group, Sinai Province, used an airport insider to smuggle a bomb into the luggage hold of the Metrojet aircraft last on Saturday. It is feared the mole is still at large, the report said.Last night officials were investigating if any Britons allied to ISIS were involved after claims that the security services had intercepted "chatter" between extremists with London and Birmingham accents in the aftermath of the explosion, it said.As of last night at least 17,000 British tourists were still stranded in Sharm el-Sheikh. Passengers were told they would be able to depart only with their hand luggage and would have to leave all hold luggage behind for additional screening.Yesterday, Egyptian crash investigators admitted that an unexplained "noise" was heard on the cockpit voice recorder in the final second before the Metrojet airliner came down.They said "all possible scenarios" -- including mechanical failure -- were still being considered as the reason for the crash.French media, however, reported that the noise heard on the recorder was consistent with a bomb.Masri, 42, is a former scholar of the al-Azhar University in Cairo, a 1,000-year-old Sunni Muslim institution. In November 2014 his Sinai-based organisation pledged allegiance to Abu Bakr al-Baghdadi, the self-proclaimed ISIS caliph, in return for weapons, finance and potential bomb-making know-how.Sinai Province has repeatedly claimed responsibility for last weekend's plane crash -- but has officially refused to explain how it was done. A Twitter account used by the group hinted last Saturday that a bomb rather than a missile was involved.