U.S. law enforcement views the case of three teenage Colorado girls who wanted to join ISIS as a good opportunity to study how the militant group recruits young people in the West , U.S. law enforcement officials told CNN . Investigators have studied Twitter and other social media activities of the Colorado girls , particularly the eldest , who was active online discussing her gradual turn to conservative Islam . The case has yielded a trove of evidence that shows how ISIS is using Westerners already among its fold to directly communicate with new recruits via social media , the officials said . The FBI is homing in on specific online recruiters , including some believed to be in Turkey , and others in Syria and Iraq . These recruiters help provide how-to guides for Westerners who are inclined to travel and join the ISIS fight , the officials said . A law enforcement official said the recruiters were giving the teenage girls a road map of how to make it from Denver to Syria . Why is ISIS so successful at luring Westerners ? `` It 's alarming that American youths are being radicalized to such a degree they 're willing to jump into the great unknown , '' one official said . Even after identifying those recruiters , however , the next step remains difficult : trying to find a way to arrest them . Most are thought to be out of reach of U.S. law enforcement . Last month , a girl of Sudanese descent and two girls of Somali descent skipped school in the Denver area and flew to Germany with plans to continue to Turkey , authorities said . The girls , 17 , 15 and 15 , wanted to join ISIS , the militant Islamist group , authorities said . One of their parents noticed his daughter 's passport missing and contacted authorities , who stopped the three girls in Germany and sent them back to the United States . About 2,000 Westerners have gone to fight in Syria , though not all for ISIS , a CIA source told CNN last month . ISIS recruitment efforts have posed a challenge to the FBI and other agencies trying to find a way to stop such recruitment . Unlike past radicalization cases , the FBI has n't found a U.S. recruitment network connecting ISIS to recruits . Instead they 're finding , as in the Colorado case , more diffuse connections . Was arrested teen on his way to join ISIS ? U.S. officials said friends of the Colorado teens noticed them change from seemingly carefree westernized high-schoolers to devout Muslims who rejected old friends and the regular cares of girls their age . But no one called attention to it until after the girls disappeared and became the subject of news stories . Officials said that while these teens decided to travel `` on their own , '' and were not forced into anything , this case reflects the larger phenomenon of American teens being lured to fight with ISIS through social media . UK spy chief Robert Hannigan has said ISIS and other extremist groups use such platforms as Twitter , Facebook and WhatsApp to reach their target audience in a language they understand , . Their methods include making use of popular hashtags for other stories in the news , such as Ebola or the World Cup , to disseminate their message , he said . They create slick videos and have learned that showing the full extent of their brutality turns people off , he said , hence their posting of videos of beheadings that stop short of showing the moment of death . U.S. winning fight against ISIS recruiting , official says .
U.S. military detention of 3 Denver girls as chance to study ISIS recruiting methods , officials say . The girls left school and planned to fly to Turkey to join ISIL , authorities say . UK spy ##master says he uses such platforms as Twitter , Facebook and WhatsApp .