DENVER — The case of three teenage girls who possibly tried to join Islamic State militants poses vexing questions for U.S. officials about terror groups’ use of social media to recruit people inside the United States.

A Colorado school official said the Denver-area girls — two sisters ages 17 and 15, and a 16-year-old friend — were victims of an online predator who encouraged them to travel overseas and eventually to Syria.

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 under Muslim leadership, complete with promises of husbands and homes.

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 students told school officials that the girls had been discussing travel plans over Twitter, Amole said.

The girls were detained at an airport in Frankfurt, Germany, and sent home over the weekend. They were interviewed by the FBI and returned to their parents in the Denver suburb of 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.

“There’s no indication they had been radicalized in a way that they wanted to fight for ISIS,” Amole said.

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 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.