It is being widely reported by the media that Shamima Begum was smuggled into Syria by an intelligence agent for Canada.
Files seen by the BBC and others show the agent, Mohammed Al Rasheed, claimed to have shared Ms Begum’s passport details with Canada, and smuggled other Britons to fight for ISIS.
Ms Begum was 15 when she and two other east London schoolgirls – Kadiza Sultana, 16, and 15-year-old Amira Abase – travelled to Syria to join ISIS in 2015.
At the main Istanbul bus station, the girls met Al Rasheed, who would facilitate their journey to ISIS-controlled Syria.
A senior intelligence officer has confirmed to the BBC that Rasheed was providing information to Canadian intelligence while smuggling people to ISIS.
The BBC has obtained a dossier on Rasheed that contains information gathered by law enforcement and intelligence, as well as material recovered from his hard drives. He told authorities that he had gathered information on the people he helped into Syria because he was passing it to the Canadian embassy in Jordan.
The dossier shows that Ms Begum was moved to Syria through a substantial ISIS people-smuggling network that was controlled from Raqqa.
Rasheed was in charge of the Turkish side of this network and facilitated the travel of British men, women and children to ISIS.
Tasnime Akunjee, the lawyer for the Begum family, said there will be a legal hearing in November to challenge the removal of Ms Begum’s citizenship and “one of the main arguments” will be that then-Home Secretary Sajid Javid did not consider that she was a victim of trafficking.
“The UK has international obligations as to how we view a trafficked person and what culpability we prescribe to them for their actions,” he said.
Mr Akunjee said it was “shocking” that a Canadian intelligence asset was a key part of the smuggling operation – “someone who is supposed to be an ally, protecting our people, rather than trafficking British children into a war zone”.
“Intelligence-gathering looks to have been prioritised over the lives of children,” he said.
Shamima Begum is now held in a detention camp in north-east Syria after her citizenship was taken away in 2019.
A Canadian Security Intelligence Service spokesman said he could not “publicly comment on or confirm or deny the specifics of CSIS investigations, operational interests, methodologies or activities.”
A British government spokesperson said: “It is our long-standing policy that we do not comment on operational intelligence or security matters.”