A Canadian intelligence agent helped Shamima Begum get into Syria after she escaped the UK and joined the Islamic State.
According to documents obtained by the BBC, he claimed to have smuggled other Britons to fight for IS and given Canada Ms. Begum’s passport information.
The loss of Ms. Begum’s citizenship is being contested by her attorneys on the grounds that she was a victim of trafficking.
Canada and the UK declined to respond to questions about security.
Ms Begum was 15 when she and two other east London schoolgirls – Kadiza Sultana, 16, and 15-year-old Amira Abase – traveled to Syria to join the IS group in 2015.
At the main Istanbul bus station, the girls met Mohammed Al Rasheed, who would facilitate their journey to IS-controlled Syria.
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, which provide extraordinary detail about how he operated.
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.
Rasheed, who was arrested in Turkey within days of smuggling Ms Begum to IS, told authorities he had shared a photo of the passport the British schoolgirl was using.
The Metropolitan Police were searching for her, although by the time Canada received her passport details, Ms Begum was already in Syria.
The dossier shows that Ms Begum was moved to Syria through a substantial IS people-smuggling network that was controlled from the group’s de-facto capital in Raqqa.
Ms Begum told the BBC’s forthcoming I’m Not A Monster podcast: “He organized the entire trip from Turkey to Syria… I don’t think anyone would have been able to make it to Syria without the help of smugglers.
“He had helped a lot of people come in… We were just doing everything he was telling us to do because he knew everything, we didn’t know anything.”
Rasheed kept information about the people he helped, often photographing their ID documents or secretly filming them on his phone.
One recording shows Ms Begum and her friends getting out of a taxi and into a waiting car not far from the Syrian border.