Kadiza Sultana, 17, from east London traveled to Syria in February 2015 with her friends Shamima Begum and Amira Abase, who are both around 15.
The schoolgirls were feared to have been radicalized by Daesh’s online propaganda.
Previously released CCTV images appeared to show the trio passing through a coach station in the Turkish city of Istanbul en route to the country's south east. They reportedly proceeded to travel onward to the Syrian border.
British media reported that Sultana had grown disillusioned with life under ISIS and was planning to flee back to Britain.
However, Sultana’s family, who had been in contact with her by phone, said they believe she was killed in a Russian airstrike back in May, according to ITV News.
“I don't have a good feeling. I feel scared... You know the borders are closed right now, so how am I going to get out?” Sultana told her sister before her death, according to a recording of the phone conversation between the siblings, obtained by ITV News.
Abase and Begum, the other two girls, are thought to still be in Syria's northern city of Raqqa.
British authorities estimate that at least 800 UK nationals have traveled to Syria and Iraq to fight alongside the terror groups operating in those countries.
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