DENVER (AP) — School officials say three teenage girls under investigation for possibly trying to join Islamic State forces in Syria were victims of an online predator who encouraged them to travel to Syria.

U.S. officials on Wednesday were still trying to determine how they made it to Europe unnoticed and whether their case is a sign of terrorism's deepening appeal among youth.

The girls, two sisters ages 17 and 15, and their 16-year-old friend, were detained in Germany and sent back to their homes in the Denver area over the weekend.

Tustin Amole, a spokeswoman for the district where they attend high school, says classmates saw the girls chatting on Twitter about their plans. Administrators are being vigilant for signs of similar intentions among other students.