Police investigating the disappearance of three British schoolgirls said Tuesday they believe the teenagers are no longer in Turkey and have crossed into Syria — likely joining dozens of other young women leaving Europe to join extremists.

The disappearance of the three British girls, aged 15 to 16, underlines fears that growing numbers in Britain and Europe are lured by online propaganda to join the Islamic State in Iraq and Syria group and become so-called "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 — all described as "straight-A students" from a highly-regarded London school — went missing from their homes on Feb. 17. Authorities say they boarded a plane to Istanbul.

The families of Shamima Begum, 15, Kadiza Sultana, 16, and Amira Abase, 15, have implored them to return home.

A fourth girl from the school where the missing girls studied disappeared in December and was thought to have left for Syria. Police said detectives investigating that case spoke to the three girls at the time, but there was nothing to suggest they were at risk of radicalization.

Expecting 'sisterly adventure'

Experts say most of the ISIS group's recruitment of young girls is conducted online on social media such as Facebook, and those trying to make the journey invariably receive advice on how to conceal their tracks.

Ross Frenett, a researcher at the London-based Institute for Strategic Dialogue, said women living in western Europe want to join the ISIS group for many of the same reasons motivating men: A vision of an Islamic utopia, a way to address perceived atrocities carried out against Muslims worldwide.

"They believe they're going to a place where they will be empowered and come of age. There is a real sense of sisterly adventure, an idealistic view of what you're going to get into and a sense of camaraderie," he said.

"An awful lot of them expect adventure but what they get is drudgery, often domestic drudgery."

The case has raised questions about whether British officials were doing enough to tackle radicalization and prevent young converts from travelling to Syria.

Turkey 'can't read the minds of tourists'

A top Turkish official complained Monday that British officials waited three days before seeking help in the case, losing valuable time.

Turkey's deputy prime minister said the girls arrived in Istanbul as tourists, and British authorities did not share enough information for Turkey to act quickly.

"It is a condemnable act, a shameful act that a country like Britain ... did not follow [the girls] closely," Bulent Arinc told reporters in Ankara, the capital. "They woke up three days after the fact to notify us."

"We don't have a mechanism that allows us to question or read the minds of tourists," he added.

The Metropolitan Police disputed that account, however, saying Tuesday that they notified the Turkish embassy in London a day after the girls went missing.