The girls, two of them 15 and one 16, have not been seen since Tuesday, when, police say, they took a flight to Turkey.
One has been named as Shamima Begum, 15, who may be traveling under the name of 17-year-old Aklima Begum, and a second as Kadiza Sultana, 16. The third girl has not been named at the request of her family, police said.
