Two 16-year-old girls who are believed to have entered war-torn Syria are "posing a threat to themselves", a counter-terrorism chief has said.
Twins Salma and Zahra Halane, who have 28 GCSEs between them, disappeared from their family home in Manchester last month when they flew out to Turkey and are now said to have crossed the border.
The Muslim youngsters, said to be "deeply religious", have ignored their family's pleas for them to come home and told them they have no intention of returning.
It is understood an older brother from the large Somalian family has joined the extremist militant group Islamic State of Iraq and the Levant (Isis), and one line of inquiry is that the girls may have travelled to join him.
The girls' parents discovered the twins were missing they they discovered their empty beds in Chorlton on June 26.
The North West Counter Terrorism Unit (NWCTU) is now investigating their reason for travel and whether they were assisted.
NWCTU head Detective Chief Superintendent Tony Mole, said: "First, I want to stress that the welfare of these two teenagers is our overarching priority.
"Two 16-year-old girls left the UK on June 26 and we believe that they have since entered Syria.
"Since this story broke in the media at the weekend there has been a lot of speculation about why the girls left the country.
"At this stage we don't know for sure why they are there, or exactly who they are with.
"As this is now developing into an investigation, the North West Counter Terrorism Unit is leading on this inquiry.
"This investigation now has two main strands - to safeguard the girls wherever possible, and to assess any further lines of inquiry as the investigation progresses.
"They are clearly posing a threat to themselves and potentially the community and their family and friends are concerned for their well-being.
"This is why the National Prevent Programme is key. Our Prevent officers work in the community - out there in the real world, dealing with real lives - to help those at risk of being radicalised or influenced into travelling to a volatile and dangerous region.
"It is also important that we thank the community for their assistance in this matter while reminding them and the wider media that the family remain very concerned for the safety of their daughters.
"If you are concerned about a friend or family member who has gone missing, or if you have any information that could help, you should contact the police via the 101 non-emergency number.
"If you see or hear anything that could be terrorist-related, trust your instincts and call the Anti-Terrorist Hotline on 0800 789 321."
The Sun newspaper reported that Salma and Zahra had achieved 28 GCSEs between them and published a photograph of one of them at a recent information day at Connell Sixth Form College in Manchester.
A spokeswoman for the college in Beswick said it would not comment.
Police are providing support to the girls' family, who also do not wish to comment.