LONDON — The voice on the end of the line sounds nervous and alone. “I don't have a good feeling,” Kadiza Sultana can be heard telling her sister. “I feel scared.”

The 17-year-old was one of three schoolgirls who left east London to join the Islamic State in Syria in 2015 after being radicalized online. New details have now emerged of the teenager's desperate wish to return home and her family's attempts to help her escape.

But it was a dream that was never realized. This week, news emerged that Sultana is believed to have been killed in a Russian airstrike in May.

“There's nothing worse than finding out that your sibling or your family member has been killed,” the family's lawyer, Tasnime Akunjee, said. “By all accounts, she was a young girl with a very, very promising future. It's a great loss to us all, really. Every effort was made from the very beginning to try and avoid this fateful news.”

Sultana, a British national of Bangladeshi heritage, was just 16 when she left her home with two 15-year-old school friends, telling her parents she would be back soon. They were all A students at Bethnal Green Academy, and their disappearance caused widespread shock.

They first flew to Turkey and then were captured on a surveillance camera catching a bus to the Syrian border.

From there, details about their lives became scarce.

They were all reportedly swiftly married to Islamic State fighters. Sultana's husband, an American national of Somali origin, is believed to have been killed late last year. The girls made sporadic contact with their relatives in London, and at first said they were enjoying their new lives, though it appears they soon became disillusioned.

The phone conversation between Sultana and her sister, Hamila Khanom, was filmed by British broadcaster ITV News. In it, the siblings can be heard discussing ways to smuggle Sultana out of Islamic State-controlled territory in a taxi.

When Khanom asks her sister how confident she feels about her chances of escape, she replies: “Zero.”

“You feel scared,” Khanom says. “Why do you feel scared?”

“You know if something goes wrong, like, that's it,” Sultana replies. “You know the borders are closed right now, so how am I going to get out?”

At one point, there is also a poignant reminder that Sultana was still a teenager. “Where's Mom?” she asks. “I want to speak to Mom.”

The fate of the other two girls with whom Sultana traveled to Syria, Amira Abase and Shamima Begum, is unknown.