Police investigating the disappearance of three British schoolgirls said Tuesday they believe the teenagers are no longer in Turkey and have crossed into Syria. The girls, aged 15 to 16, disappeared from their London homes on Feb. 17 and boarded a flight to Istanbul in Turkey. Authorities believe the girls could be on their way to join the Islamic State group. The case underlines fears that growing numbers of young women in Britain and Europe are lured by online propaganda to join extremists and become “jihadi brides.” Security officials say at least 500 Britons have travelled to Syria to fight with extremists, often via Turkey. Experts estimate about 50 are female.