DENVER (AP) — The case of three teenage girls being investigated for trying to join Islamic State militants poses vexing questions for U.S. officials about the use of social media by terror groups to recruit people inside the United States, experts said Wednesday. 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 in the caliphate," complete with promises of husbands and homes. "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.