Three British teenage girls suspected of traveling from London to Turkey with intentions to join the self-proclaimed group Islamic State of Iraq and Syria (ISIS) are now believed to have crossed over into their destination country Syria, according to London’s Metropolitan Police on Tuesday.

In a police statement, officials said that they “now have reason to believe that (the girls) are no longer in Turkey” and have crossed into neighboring Syria, parts of which have been taken over by the Islamist terror group. British and Turkish authorities have been working to find the teens in Turkey before they could cross the Turkish-Syrian border.

British police announced last week that three London classmates, Shamima Begum, 15; Kadiza Sultana, 16; and Amira Abase, 15; had boarded a Turkish Airlines plane bound for Istanbul on February 17, from the London Gatwick Airport. The next day, police contacted foreign liaison officers at the Turkish Embassy in London, confirmed by an official of the Scotland Yard. A British counterterrorism officer said police had been “reaching out to the girls using the Turkish media and social media” in an effort to persuade them to return home. The parents of the girls also publicly begged for them to come back home.

It is reported that reported that the girls had been smuggled into Syria from Turkey four or five days ago near the Kilis border crossing. Turkey, which has been accused by its Western allies of failing to do enough to stop jihadists crossing into Syria from its territory, had earlier accused Britain of failing to provide information about the girls sooner. Turkey has become the most well known and traveled route for those trying to join Syria’s factions since an anti-government uprising erupted in Syria more than three years ago.

In the days before the three girls left for Turkey, at least one allegedly contacted a young woman, Aqsa Mahmood, accused of recruiting volunteers for ISIS using social media after she left her home in Scotland to travel to Syria in 2013. The Mahmood family released a statement over the weekend in which they condemned their daughter’s actions, “Your actions are a perverted and evil distortion of Islam,” but they also said the UK security services “have serious questions to answer” over their failure to become involved after one or more of the missing girls contacted Aqsa using Twitter.

A spokesman for the Turkish President said British and Turkish security forces and intelligence agents are nworking together in an effort to locate the three British girls.