The group, whose leader Mohammed Emwazi gained notoriety for appearing in videos showing the murders of Western hostages, are said by former hostages of ISIS to have been assigned to guard foreign prisoners, and were nicknamed "the Beatles" due to their English accents.
Londoner Emwazi, referred to by British media as "Jihadi John", is believed to have been killed in a US drone strike last year.
The Washington Post and BuzzFeed said Kotey was a soccer fan who had grown up in West London and converted to Islam in his early 20s, after meeting a Muslim woman with whom he had two children. He had attended the same mosque as Emwazi, they reported.
Kotey has a Greek Cypriot mother and Ghanaian father, and left Britain in 2009 to travel to Gaza as part of an aid convoy, the report said. The Washington Post said a US intelligence official had confirmed that Kotey had travelled to Syria.
Reuters was unable to immediately verify the reports.
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