The Denver-area girls — two sisters ages 17 and 15, and their 16-year-old friend — were detained at the airport in Frankfurt, Germany, and sent home over the weekend. They were interviewed by the FBI and returned to their parents in suburban Aurora. Those in the tight-knit east African community where they live said the sisters are of Somali descent and their friend is of Sudanese descent.
The episode posed vexing questions for U.S. officials, including about the use of social media by terror groups to recruit people inside the United States and what can be done about it.
"Social media has played a very significant role in the recruitment of young people," said FBI spokesman Kyle Loven in Minneapolis, home to the largest Somali community in the U.S. Authorities there have been concerned about terror recruiting of the young for years.
"What it indicates is we have to be really careful about people in impressionable years and what they're doing on the Internet," said Jim Davis, former special agent in charge of the FBI in Denver.
At least one of the girls was communicating with someone online who encouraged the three to travel to Syria, said Tustin Amole, a spokeswoman for the Cherry Creek School District where the girls attend high school.
Fellow high school students told school officials on Monday that the girls had been discussing travel plans over Twitter, Amole said.
"There's no indication they had been radicalized in a way that they wanted to fight for ISIS," Amole said, adding that the students had no prior problems aside from unexcused absences on Friday. She did not elaborate.
A U.S. official said evidence gathered so far made it clear that the girls were headed to Syria, though the official said investigators were still trying to determine what sort of contacts they had in that country. Another U.S. official said that investigators were reviewing evidence, including the girls' computers. The officials spoke on the condition of anonymity because they were not authorized to discuss the ongoing investigation by name.
Still unknown is how the girls managed to get to Frankfurt. The U.S. government doesn't have any restrictions on children flying alone, domestically or internationally. Airline policies vary.
Most U.S. airlines allow children 12 and older to fly alone but often with restrictions on international flights, according to the U.S. Transportation Department.
The girls' parents reported them missing Friday after they skipped classes. They had taken passports and $2,000 in cash.
At some point, the U.S. informed German authorities at the airport about the girls arriving alone on their way to Turkey, German Interior Ministry spokeswoman Pamela Mueller-Niese told reporters Wednesday. She said the three were detained by German police, with approval from a judge, and returned voluntarily to the U.S. on Sunday.
In Denver, the FBI interviewed the girls before sending them home. Once there, the girls told a deputy they stayed in the Frankfurt airport for an entire day. They said they had gone to Germany for "family," but wouldn't elaborate.
Secretary of State John Kerry, speaking during a visit to Berlin on Wednesday, said the case was "an example of good cooperation between us and the increased vigilance of law enforcement on this issue of the movement of people from one country to another."
Amole said the school district was being "extra vigilant" in light of the FBI's concerns that the girls' friends or classmates might have similar intentions.
Mohamed Nur, head of the Somali Community Center of Colorado, said residents want to know more about what really happened so they can find ways to "do a better job and show kids a good path."
Terror recruiting has been a problem for years in Minneapolis. Since 2007, roughly 22 young Somali-Americans have traveled to Somalia to take up arms with al-Shabab, an al-Qaida linked group. Those were all men.
Within the last year, a handful of people from the community left Minnesota to join militant groups in Syria, and this time, there are fears that women might have been targeted. Loven said the FBI is working with the Somali community to establish trust and help identify young people at risk for radicalization.
In Colorado, Shannon Conley, a 19-year-old nurse's aide, pleaded guilty in September to planning to help the Islamic State after trying to board a flight in Denver to get to Turkey. Conley planned to marry a man she met online who said he was fighting with the militants.
Mia Bloom, a professor of security studies at the University of Massachusetts-Lowell, said the girls' story so far suggests how Islamic extremists have mastered social media to prey on younger and younger women with "Disney-like versions of what it is like to live in the caliphate."
A spokesman for the U.S. attorney's office in Denver would not say whether prosecutors plan to charge the girls with a crime. State prosecutors said they have no imminent plans to charge the girls. Amole said they will not face discipline.
"Our biggest concern is for the safety and well-being of these girls," Amole said.