According to The Times, British counter-terrorism agencies are investigating documents that give permission to IS fighters holding UK passports to leave the region. This raises fears that they could be plotting attacks in Britain.
The so-called leaving permissions or exit cards, found among thousands of documents obtained by the Syrian website Zaman Al Wasl, include a British Iraqi dubbed Abu Bakr al-Iraqi, whose commander gave him permission to leave for "work" in July 2014.
The exit cards, only a few of which were disclosed to the newspaper, are thought to provide a bureaucratic vouchsafe for fighters to be able to safely cross the ISIS border as the group is known to execute deserters.
The documents leak, which covers the very early days of IS rule in northern Syria and some of northern Iraq, first emerged last month and exposed the meticulous detail with which IS administers its recruitment process.
Other than "work", the cards included fighters leaving for "family circumstances" and "meeting his mother". Saeed Hamid, an IS fighter from Birmingham, was given permission to leave but appears to have not returned to the UK before being killed.
The family of the 21-year-old were later sent a picture of his dead body. They have been cooperating with UK counter-terrorism police and security services with their investigations.