Kadiza Sultana's family lawyer Tasnime Akunjee told BBC last night that they heard a report of her death in Raqqa, an ISIS stronghold in Syria, a few weeks ago.
"The family are devastated. A number of sources have said that she has been killed and she has not been in contact with the family for several weeks. Over a year ago, she had been talking about leaving. There was a plan to get her out," Akunjee said.
All three were pupils of Bethnal Green Academy in east London and had told their parents they were going out for the day.
"We were expecting this, in a way. But at least we know she is in a better place," Sultana's sister Halima Khanom said in a statement in reference to her feared killing.
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Akunjee said though Sultana had expressed a desire to return to the UK but feared of "brutal" consequences from ISIS.
"I think she found out pretty quickly that the propaganda doesn't match up with the reality."
The schoolgirls are among more than 800 Britons who are believed to have left the UK to join ISIS or other militant groups in Syria and Iraq.
In March last year, the Scotland Yard commissioner, Sir Bernard Hogan-Howe, said the teenagers could return home without fear of being prosecuted for terrorism, as long as no evidence emerged of them being engaged in violence.
A spokesperson for the UK Foreign Office said: "The UK has advised for some time against all travel to Syria. As all UK consular services there are suspended, it is extremely difficult to confirm the status and whereabouts of British nationals in Syria.
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