Updated at 7:23 a.m. ET.

Three teenage girls from the Denver suburbs were taken into custody by German authorities over the weekend at Frankfurt airport while trying to travel to Turkey, U.S. officials reported on Tuesday, according to The Associated Press.

Citing unnamed officials, the AP and CNN report that investigators in the case believe the girls may have been trying to join a wave of foreigners from dozens of nations who have used Turkey as a route into Syria to fight with the Islamic State, also known as ISIS.

Police in Arapahoe County had originally treated the girls' disappearance as a standard runaway case, Colorado Public Radio's Megan Verlee tells our Newscast Desk. Two of the girls, age 15 and 17, are sisters. Their father had reported his daughters missing, along with their passports and $2,000. The third girl, age 16, is an acquaintance of the sisters. Her father reported her missing when the school called to say she had not been in class that day.

The three girls are now home with their families in Aurora, Colo. It is not yet clear whether they will face any charges.

The 16-year-old's family is from Sudan and the sisters are of ethnic Somali origin, according to The Denver Post. (The Post has also posted the Arapahoe County Sheriff's report on the case.) Reuters reported that Colorado has a large population of refugees from Somalia who work in meatpacking plants.

The girls are not the first young women from Colorado this year to be stopped while attempting to travel overseas to join the Islamic State. A 19-year-old from Arvada, Colo., pleaded guilty last month to conspiracy charges after trying to travel to Syria in April. She planned to meet and marry a Tunisian man there who claimed to be fighting with the Islamic State. The two had met online.

FBI Director James Comey said in an interview on the CBS News broadcast 60 Minutes that the American government knows of about a dozen U.S. citizens fighting alongside militants in Syria. Comey said a big concern with these citizens is what they plan to do once they return home.

"Ultimately, an American citizen, unless their passport's revoked, is entitled to come back," says Comey. "So someone who's fought with ISIL, with an American passport wants to come back, we will track them very carefully."

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