DENVER — Authorities are investigating whether three teenage girls from suburban Denver who may have been trying to join Islamic State militants in Syria have friends or associates with similar intentions.

A U.S. official said the evidence gathered so far made it clear that the girls — two sisters, ages 17 and 15, and their 16-year-old friend — were headed to Syria, though the official said investigators were still trying to determine what sort of contacts they had in that country. The official said investigators would be trying to figure out whether there were “like-minded” friends and acquaintances in the girls’ social circle.

The sisters are of Somali descent, and their friend is of Sudanese descent, The Denver Post reported.

FBI agents stopped the girls at the Frankfurt, Germany, airport over the weekend. They were then sent back to Colorado where they were safe and reunited with their families, FBI spokeswoman Suzie Payne said.

The official said the girls were headed toward Turkey en route to Syria and that investigators were now 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.

The U.S. attorney’s office in Denver declined to comment. It’s unclear whether the girls will face charges.

Authorities have not said how they think the girls became interested in helping the Islamic State militants. In Conley’s case, she told agents she wanted to marry a suitor she met online who said he was a Tunisian man fighting with the Islamic State in Syria.

Conley said she wanted to use her American military training with the U.S. Army Explorers to fight a holy war overseas, authorities said.

If she could not fight with the extremists, she told agents, she would use her training as a nurse’s aide.