One of the three schoolgirls believed to be heading to Syria to join Islamic State militants flew to Turkey on her older sister's passport, it has been confirmed.
Shamima Begum, 15, used the passport of her 17-year-old sister Aklima to leave the UK on Tuesday.
The other girls are Amira Abase, 15, and Kadiza Sultana, 16. They are pupils at Bethnal Green Academy in London.
UK police officers have gone to Turkey but their role has not been confirmed.
A Scotland Yard said: "Officers are working closely with the Turkish authorities who are providing a great deal of assistance and support to our investigation."
Security services have been criticised after it emerged that - before leaving the UK - Shamima sent a Twitter message to Aqsa Mahmood, who left Glasgow for Syria in 2013 to marry an Islamic State fighter.
According to a lawyer for Ms Mahmood's family, her Twitter account has been "monitored" by police since she left Britain - and he said authorities should have seen Shamima's message and taken action before she and her two friends followed.
The girls were all studying for their GCSEs at the east London school, which reopened on Monday after half term.
Dr Muhammad Abdul Bari, from the East London Mosque and a former teacher at Bethnal Green Academy, said all the parents "were shocked".
"Children running away from home or disappearing is every parent's nightmare," he told BBC Breakfast.
He said the girls were "talented and dynamic" but "impressionable" and that they could end up "with the most vile and dangerous group on earth" was "heartbreaking" for the parents.
The message from the school would be one of unity and that every pupil's safety was the priority, he said.
Bob Milton, former Met Police commander, said there was a counter-radicalisation programme at the school and the girls had been spoken to.
"They had satisfied the person that they were no longer at risk but that clearly wasn't the case," he said.
How they left the country was a different matter and the lack of exit control was "shameful", he added.
The girls boarded a Turkish Airlines flight from Gatwick, and would have needed a visa for Turkey and a passport.
Children between seven and 12 years old can travel alone on Turkish Airlines but declaration forms must be filled in at the check-in counters and signed by parents or guardians.
The Archbishop of Canterbury Justin Welby has spoken about the girls, saying it was a "nightmare situation" for the families.
The archbishop, who was addressing business and church leaders on community cohesion in Birmingham, said: "It's a deception, isn't it - and the families are absolutely devastated by it."
The families of the girls have made appeals for them to come home.
Shamima's sister, Renu Begum, said she hoped her sister had gone to Syria to bring back another girl from Bethnal Green Academy who went there in December.
Ms Begun said Shamima and her friends were "young" and "vulnerable" and if anyone had tried to persuade them to go to Syria it was a "cruel and evil" thing to do.
"We don't want her to do anything stupid - she is a sensible girl," she added.
"We just want her home, we want her safe."
Amira's father, Abase Hussen, said: "The message we have for Amira is to get back home. We miss you. We cannot stop crying. Please think twice. Don't go to Syria."
Holding a teddy bear Amira had given her mother, Mr Hussen added: "Remember how we love you. Your sister and brother cannot stop crying."
In an appeal to Kadiza, her sister Halima Khanom said: "Find the courage in your heart to contact us and let us know that you are safe and you are OK. That is all we ask of you."