LONDON — 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.

The three girls in the latest case have all been described as “straight-A students” from a highly-regarded London school. The families of Shamima Begum, 15, Kadiza Sultana, 16, and Amira Abase, 15, have implored them to return home.

“They appear to have been induced to join a terrorist group that carries out the most hideous violence and believes girls should be married at nine and women should not leave the home,” Prime Minister David Cameron said Monday.

As officials continued searching for the teenagers, a top Turkish official complained that British officials waited three days before seeking help in the case, losing valuable time.