The girls, who are from the Denver area, were trying "to fulfill what they believe is some vision that has been put out on a slick media campaign" by radical groups in Syria, including ISIS, the official said.
The girls have not been identified, but law enforcement officials said two are sisters -- 15 and 17 years old -- and one is 16 years old. The two sisters are from Somalia and the 16-year-old is from Sudan, according to a federal law enforcement source briefed on the case.
This is part of a worrisome trend of "disaffected youth" who are being radicalized, the official said, pointing to the wider phenomenon of foreign fighters converging on the conflict in the region.
There have been other cases of high school age kids trying to get to Syria that haven’t been made public, the official said.
The girls were persuaded to travel overseas and were in contact with someone in Germany, the official said, noting that someone close to them notified authorities.
They were turned around and sent back to the United States, the official said, noting that because they are minors it remains unclear whether they will be charged with anything or be "arrested."
The Arapahoe County Sheriff’s Department in Colorado responded to a call on Oct. 17 of three missing teens, a department spokesman said, noting that deputies responded to two homes with Denver mailing addresses that are technically in unincorporated Arapahoe County. Family and friends reported the girls missing, but nobody mentioned or knew that the girls had gone overseas, according to the sheriff's department. The case was closed when a family member called Monday to tell the sheriff's department that the girls were back safe in Colorado, a spokesman said.
"FBI Denver Division is aware of the situation and assisted with bringing the individuals back to Denver. The juveniles are safe and reunited with their families," FBI Denver spokeswoman Sue Payne said.