Somali teen Yusra Hussien, 15, is thought to be in Turkey with a 17-year-old girl from London.
She vanished from her Bristol home last Wednesday after failing to meet her father after school.
Yusra is thought to have met the older girl in London and boarded a plane to Istanbul from Heathrow.
Schoolmates described her as an A-grade student who wanted to be a dentist.
Police chiefs yesterday said they feared the youngster had been “radicalised” while a family friend said she feared Yusra had been influenced by online Islamist extremism.
Relatives of the schoolgirl gathered at the family’s terraced home in the Easton area of Bristol yesterday.
Her mother Safiya confirmed her daughter was “missing” and another woman who said she was an aunt added: “We are all really confused.”
The girl’s father Mohammed is believed to be a youth worker but did not wish to speak yesterday at their home just off Bristol’s Stapleton Road, at the heart of the city’s Muslim community.
A friend close to Yusra’s family yesterday said she spent the day on Monday with Yusra’s parents as they struggled to understand what she had done.
She said: “It wasn’t the fact that the mother could not communicate with her daughter, it wasn’t because the home was a broken home, it wasn’t because the young girl was a loner.
“But what it all seemed to point to was the main brainwashing machine – the internet.
“An entity that has no barriers, that has no such alarms that go off when a young person may be involved in any such discussion of extremism. The use of media and social media is something that no parent or family can monitor closely.
“Who their children talk to, which sites they are on, who befriended who and where, what meetings are being arranged, this information can all be deleted from a device’s history, and in today’s day and age the majority of children are tech savvy.”
She added that the girl’s parents were both respected members of the community.
Anira Khokhar, who spoke on behalf of the Hussien family yesterday, said they have “lost a daughter” and believe she is “in danger”.
She said: “At this present moment they are just a family whose daughter is somewhere that they are unaware of and she is in danger. The family is very distraught.”
Speaking of Yusra’s sisters and brother, she added: “We have to ensure that as a community – a British community – we all come together and unite and protect those young kids. The last thing they want to see about their sister are words such as ‘jihadist bride’ or ‘radicalisation’ or ‘extremism’ because that’s not the case at this present moment.”
Friends outside the City Academy in Easton said Yusra, who had a keen interest in science, had just started year 11.
Classmates described the schoolgirl as “very religious” and one girl added: “She had a very good education, she was very smart. She was very intelligent and had a very bright future ahead of her. I hope she is OK. I hope we hear some good news soon.”
Avon and Somerset Police Assistant Chief Constable Louisa Rolfe said the girl “may have been radicalised”. She said: “We can confirm that a 15-year-old from Bristol has travelled to Turkey and we understand she may be attempting to make her way to Syria.
“There are indications she may have been radicalised but at the moment our priority is to find her before she crosses the border to Syria and make sure she is safe.
“We must all be vigilant and ready to spot the signs of radicalisation.”
Ms Rolfe added: “Often, young Muslims who go to Syria can be naive and don’t recognise that they are being sucked into joining extremist groups.
“This is not about criminalising these young people, it’s about preventing tragedies.”