Kadiza Sultana, then 16, and two other teens boarded a flight from London's Gatwick Airport to Istanbul on February 17, 2015. They were thought to have crossed the border from Turkey into Syria within days.
Tasnime Akunjee, an attorney who represents Sultana's family, said the family received news of her death in Raqqa "some weeks ago" -- at the same time communication from her stopped. Akunjee said he had been working with the family to try to bring Sultana back to London.
Reports of Sultana's death have not been independently confirmed.
The UK Foreign Office said: "It is extremely difficult to confirm the status and whereabouts of British nationals in Syria as all UK consular services there are suspended."
Sultana left the UK with east London classmates Shamima Begum and Amira Abase, both 15.
A packing list they left behind looked like one any teenage girl might make before leaving on vacation: makeup, bras, underwear and boots, according to The Guardian newspaper, which saw the list.
Girls married men 'approved by Isis'
Two of the three girls were believed to have married men approved for them by ISIS, according to The Guardian
, which cited family members in its report.
Akunjee, the lawyer, could not verify the ISIS connection, but told CNN
the girls were believed to have married "people in the territory of Syria. ... We don't know if they are ISIS fighters."
One of the girls called her family to tell them she was married, he said, while the other girl revealed the news on social media.
"I don't have a good feeling"
The three girls were believed to be living in the ISIS-controlled city of Raqqa. The ISIS stronghold has come under fire from both the Syrian army backed by Russian air power, and a Kurdish-led alliance supported by U.S. airstrikes, in an attempt to reclaim the city.
The bombing has also made communication with the girls almost impossible for their families, said Akunjee.
When Sultana's sister spoke to her over the phone in December 2015, the teenager told her the man she had married in Syria had been killed and that she wanted to come back to London, according to CNN affiliate ITN.
"I don't have a good feeling, I feel scared," Sultana is heard saying over the phone, adding "You know the borders are closed right now so how am I going to come out?"
"I'm not going through PKK territory to come out. I'm never going to do that ever," she said, referring to regions controlled by the militant Kurdistan Workers' Party.
The family is still trying to get Sultana out of Syria -- though now they must face the prospect she may not be alive.