A spy for Canada is accused of smuggling a teenager into Syria in 2015, after she fled the United Kingdom to join the Islamic State group, according to BBC News.

Shamima Begum, then 15, was one of three teenage girls reported missing from an East London neighbourhood in 2015 after they allegedly fled to join ISIS.

Unbeknownst to them, the man who helped get them across the border into Syria was a double agent, working as a foreign operative for the Canadian Security Intelligence Service (CSIS), the BBC says.

Information gathered by international law enforcement and intelligence implicates Mohammed Al Rasheed with smuggling Begum and other British citizens into Islamic State territory while working as an informant.

According to documents obtained by the BBC, Al Rasheed went to the Embassy of Canada in Jordan to try to apply for asylum in 2013, and was told he would receive Canadian citizenship if he agreed to collect information about the activities of ISIS.

He then began helping people enter Islamic State territory in Syria from Turkey, while sharing their information with Canadian intelligence. He claims to have helped 15 people from western countries travel into ISIS territory, while being paid by Canada to share intel on the flow of foreign fighters.

He operated as part of an ISIS people-smuggling network for approximately eight months before he allegedly helped Begum enter Syria, the BBC reports. He was arrested in Turkey days later.

Begum has since said she was at a "vulnerable point" in her life, and that "it was easy for people to take advantage of (her)."

She is now in a detention camp in Syria and wants to be allowed to return to England. The British Home Office denied her entry and stripped her of her citizenship, citing security risks.

Unable to return home, the BBC reports, she is still being held in a detention camp in northeast Syria.

In light of these allegations, Tasnime Akunjee, the lawyer for Begum's family, told the BBC the family plans to challenge the removal of her citizenship on the grounds that the British government did not recognize her as a victim of human trafficking when it revoked her citizenship.

"The U.K. has international obligations as to how we view a trafficked person and what culpability we prescribe to them for their actions," he told the BBC.

Akunjee said he was shocked that a Canadian intelligence asset was 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," he told the BBC.

"Intelligence-gathering looks to have been prioritized over the lives of children."

Exactly what Canada knew and shared with British authorities about the whereabouts of Begum and her friends is now embroiled in controversy and making headlines overseas.

In a news conference Tuesday, Prime Minister Justin Trudeau didn't deny Ottawa's involvement with the operative. He said that while the Canadian intelligence services need to be "flexible" and "creative" in their fight against terrorism, they are bound by strict rules.

"There are rigorous oversight mechanisms that are in place with the clearances necessary to look into the operations and the decisions taken by intelligence services in their work to keep Canada and Canadians safe in a very dangerous world," he told reporters.

"I know there are questions about certain incidents or operations of the past and we will ensure to follow up on this."

It's alleged that Canadian officials and British intelligence withheld information when the Metropolitan police in London started searching for Begum and her friends.

If that's the case, it was for good reason, according to Mubin Shaikh, who previously worked as an undercover source with CSIS.

"If he would have been exposed … ISIS would have sawed his head off on camera for everyone to see to dissuade other spies from trying to work with western nations," the counter-terrorism expert told CTV National News Wednesday.

Asked how Shaikh believes CSIS would have viewed this informant, the former source said: "certainly a treasure trove is what he produced, not just for the Canadians but for other intelligence agencies as well."

CTV News reached out to CSIS for comment, but the agency declined to respond.

Correction:

This story has been corrected to reflect a transcription error in a updated version of the original story.