A missing teenage girl from Bristol may be attempting to get to Syria, police have said.
Detectives have been tracing the 15-year-old's movements from the moment she left her Easton home to her arrival in Istanbul, Turkey.
Assistant Chief Constable Louisa Rolfe said there were indications the girl "may have been radicalised".
"Our priority is to find her before she crosses the border to Syria and make sure she is safe," she said.
"We must all be vigilant and ready to spot the signs of radicalisation.
"Often, young Muslims who go to Syria can be naive and don't recognise they are being sucked into joining extremist groups."
The girl's parents reported her missing last Wednesday, having last seen her at about 07:00 BST.
But family spokeswoman Anira Khokhar described reports she was heading to Syria as "speculation".
"They are just a family whose daughter is somewhere they are unaware of and she is in danger," she said.
"She is an intelligent, beautiful young lady who is being missed by her family and that's all that matters at this present moment."
Bristol city councillor Hibaq Jama said on the day the girl disappeared, she left for school as usual but was not there when her father went to pick her up at 15:30 BST.
Ms Jama said the family was "absolutely devastated" and "distraught", adding the teenager's actions were "completely out of character".
"Avon and Somerset Police said it had been asked to put two officers outside her school, The City Academy in Bristol, to protect pupils from the media attention.
A school spokeswoman said the school would not be commenting and that "any statements would come from the police".
The student travelled to London where she met a 17-year-old girl from Lambeth.
Both are believed to have travelled from Heathrow Airport.
Avon and Somerset Police said it was respecting family wishes by not naming the Bristol girl, adding that there would also be no "policing advantage" in doing so.
Detectives in Bristol are also being assisted by officers from the Metropolitan Police and its network of international liaison officers.
In July, 16-year-old twins flew from Manchester to Turkey and officers believed the pair had headed to Syria.
How can a 15-year-old travel alone to Istanbul?
• A number of airlines including British Airways, KLM and Lufthansa offer daily direct flights to Istanbul from Heathrow
• The Home Office and the Department for Transport both confirmed it was up to individual airlines whether to permit unaccompanied children on flights
• Regulations differ depending on the airline - British Airways offers a Skyflyer Solo service which looks after children up to the age of 12 travelling alone, older children do not have to use the service
• Aseel Muthana, 17, from Cardiff, left Britain in February to join his older brother Nasser, fighting for IS in Syria, and 16-year-old twins Salma and Zahra Halane left their Chorlton home to join the Jihadi group in the country in July
• The Foreign and Commonwealth Office (FCO) advises against all but essential travel to areas within 10km (six miles) of Turkey's border with Syria