How did ISIS recruit Colorado girls?

• U.S. using case of 3 Denver teens as chance to study ISIS recruiting methods, officials say


• The girls skipped school and planned to fly to Turkey to join ISIS, authorities say


• UK spy chief says ISIS uses such platforms as Twitter, Facebook and WhatsApp

(CNN) -- 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.

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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.

"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 hasn't found a U.S. recruitment network connecting ISIS to recruits. Instead they're finding, as in the Colorado case, more diffuse connections.

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.