A suspected associate of "Jihadi John," a prominent British ISIS terror organization member known for his appearance in beheading videos, is being held by authorities in Turkey, two senior Turkish officials said on Friday, a day after the United States targeted the militant in an air strike in northern Syria.A man thought to be Aine Lesley Davis, one of a group of British extremists believed to have been assigned to guard foreign prisoners in Syria, was detained in Istanbul, the officials said.They declined to give further details, saying investigations by the police and intelligence agencies were continuing.