Federal Bureau Investigation (FBI) agents who tracked down three Denver teens en route to Syria are investigating the possibility that the girls were attempting to join the Islamic State group, U.S. officials said Tuesday.

The girls, a 16-year-old of Sudanese descent and her Somalian sister friends, aged 15 and 17, were allegedly coaxed to join the radical group through the slick media campaign ISIS is known to operate, an unnamed law enforcement official who was briefed on the case told ABC News.

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FBI Spokeswoman Suzie Payne said agents apprehended the teens in Germany following reports of missing teenagers in Arapahoe County. They were sent back home in Denver on Sunday and were reunited with their families who first alerted local authorities of three runaway teens late Friday.

Payne declined to name the girls, but The Denver Post newspaper said they are students at the Cherry Creek School District.

Their families first reported the incident to Arapahoe County Sheriff's Department, saying the three left home without indicating where they were headed.

The bureau chief of the Arapahoe County Sheriff's Department, Glenn Thompson, said his officers received reports of missing teens, but did not bring in the FBI to the case since taking the report. The case was closed by Sunday when a family member told the police of the girls' return in Colorado.

The parents notified the FBI after realizing they have taken their passports with them, prompting a suspicion that they might be flying to Syria via Turkey, according to CNN..

It is not exactly known how the girls planned their departure, but an unnamed U.S. official told ABC News that they were in contact with someone in Germany who intended to help them join the jihadists fighting in Syria.

Pointing to the growing number of foreign fighters flying to Syria and Iraq to join terrorist organizations, the official said the Denver teens are part of a worrisome trend of Islamist groups radicalizing and recruiting "disaffected youth."

Tagsforeign ISIS fighters, Denver teens joining ISIS, FBI, ISIS