Three teenage girls from Denver, who were planning to join ISIS militants in Syria, were stopped by the Federal Bureau of Investigation (FBI) agents at Frankfurt airport in Germany. They were then sent back to Colorado to reunite with their families. Because of the incident, the US government conducted an investigation.

The Guardian reported a US official said the evidence gathered made it clear that the Denver girls were heading for Syria. The official said investigators were still trying to determine what sort of contacts they had in the country. The girls, who were allegedly joining the ISIS insurgents, were two sisters ages 15 and 17 and their 16-year-old friend.

FBI spokeswoman Suzie Payne said the agents stopped the Denver girls at Frankfurt airport in Germany and were then sent back to Colorado where they were reunited with their families. The official also stated the girls had been heading to Turkey on the way to Syria to join ISIS. According to The Denver Post, the sisters are of Somali descent while their friend is Sudanese.

As stated by The Huffington Post, the girls were reported missing after they skipped school Friday. Arapahoe County Sheriff's Department Bureau Chief Glenn Thompson said the families of the teenagers had no indication of where they might be.

Because of the incident, investigators were currently reviewing evidence including the girls' computers. And another US official called the case generally troubling to both the society and the nation. Investigators are also trying to figure out whether there were "like-minded" friends and acquaintances in the girls' social circle.

A missing person's report from the Arapahoe County contains information of the girls' activities. They stayed in the Frankfurt airport for an entire day before being detained, questioned and returned to Denver, where they were probed further by the FBI and sent home.

The Denver girls suspected of joining ISIS told authorities they had gone to Germany for family but would not elaborate.

According to police reports, suspicion arose when the sisters' father realized his daughters were gone along with $2,000 and their passports. While the 16-year-old's father became anxious when he got a call from her school saying she hadn't reported to class.

CNN reported the girls' parents said they had no idea their children planned to travel and none of them had ever run away before.

Because of the Denver girls' incident, a former CIA officer said it's more than just Islam's radical interpretation that draws teenagers to the extremely blood-thirsty ISIS militant group.

"They're often times searching for an identity, because what the jihadis are actually pushing is a specific narrative, which is: Your people (Muslims) are being oppressed in this place called Syria; your government is doing nothing; we're the only ones who are actually going to help you out," said Aki Peritz. "Why don't you join the fight?"

The Soufan Group's Richard Barrett said many of the teenagers lack a sense of belonging where they live, and they believe ISIS can give it to them. ISIS also continuously turns PR machine, making use of slick videos and social media.

Terror experts in the United States are worried about the security of the nation because of ISIS' global digital reach. Counterterrorism expert Matt Olsen said there are terrorist groups in Yemen and in Syria with stated ambitions of striking on American territory.

Since Edward Snowden revealed secret US surveillance programs, the use of the internet on tracking terrorists has become more difficult.

Meanwhile in Denver, two US officials said the 17-year-old girl was allegedly the instigator of joining ISIS and planning the trip, having it planned for months.

The officials also said the three Denver girls researched the plan online, checking out websites where ISIS extremists discuss how to get to Syria. The online activity didn't start out any tripwires the FBI normally uses to flag possible jihadist supporters.

The FBI is also combing all of the girls' communications to see if anyone was helping them. However, the parents of the teens in Denver believed ISIS was behind the trip.

On the other hand, US officials are placing new vigor into attempting to understand what radicalizes people far removed from the fight.  And since foreign fighters from several are pouring into the Middle East to join ISIS and other terrorist organizations, authorities are trying to urge nations to do a better job of keeping them from joining up the group.

The incident of the three Denver girls comes after a month when 19-year-old Shannon Conley of Arvada, Colorado, pleaded guilty to charges that she conspired to help ISIS militants in Syria.

Meanwhile, the US Attorney's Office in Denver declined to comment on the latest cases. It is also indistinct whether the girls will face charges for allegedly joining ISIS.