Twin teenage girls who are thought to have travelled to Syria are "deeply religious Muslims", police understand.
It is understood an older brother 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 flown out to join him.
The girls, from a large Somalian family, flew from Manchester International Airport to Turkey which borders Syria where the civil war involving Muslim militants is raging.
Greater Manchester Police said the pair had been in contact with relatives since their departure.
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Gunshots and explosions have been reported at Istanbul's Ataturk Airport.
Police say they are keeping an open mind about the motive and appealed for anyone with information to come forward.
Rain will clear east through this evening to leave a drier night for most, however showers will continue across some northern areas.