New details on the Islamic State group’s former detention centres for foreign hostages in Syria and on grave sites offer clues on where victims may be buried, a rights group said Thursday. The US-based Syrian Justice and Accountability Centre (SJAC) said it had managed to pinpoint the exact locations of seven detention facilities once run by the extremist group. It said it had identified three potential burial grounds after monitoring US court proceedings against a member of the notorious IS kidnap-and-murder cell dubbed “the Beatles”. The four-member cell, named after the pop band by their ca…