Three teenage girls from suburban Denver flew to Europe in a possible bid to join Islamic State militants in Syria, and now authorities are looking at their friends to see if any have 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.

Another U.S. official said the girls were headed toward Turkey en route to Syria and that investigators were 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 sisters are of Somali descent, and their friend is of Sudanese descent, The Denver Post reported.

The girls were stopped at the Frankfurt, Germany, airport over the weekend.

The U.S. informed German authorities at the airport about the girls arriving alone on their way to Turkey, German Interior Ministry spokeswoman Pamela Mueller-Niese told reporters Wednesday. She said the three were detained by German police, with approval from a judge, and returned voluntarily to the U.S. on Sunday.

They have been reunited with their families in Colorado, FBI spokeswoman Suzie Payne said.

A spokesman for German federal police at Frankfurt airport confirmed that the three girls returned to the U.S. on Sunday after they arrived the day before. He spoke on condition of anonymity in line with department rules.

Secretary of State John Kerry, speaking during a visit to Berlin on Wednesday, said the case was "an example of good cooperation between us and the increased vigilance of law enforcement on this issue of the movement of people from one country to another."

The girls' fathers reported them missing after they skipped school Friday, taking their passports and, in the sisters' case, $2,000 in cash.

But the families had no indication of where they might have gone, said Glenn Thompson, bureau chief of the Arapahoe County Sheriff's Department, whose officers took the missing persons report.

The report contains details of the girls' movements.

Upon their return, they told deputies they stayed in the Frankfurt airport for an entire day before being detained, questioned and returned to Denver, where they were further questioned by the FBI and sent home.

They told authorities they had gone to Germany for "family" but wouldn't elaborate.

The families said they had no prior problems with the girls. A man who answered the door at the sisters' home in the Denver suburb of Aurora identified himself as a family member but said he had no comment.

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

Crimes committed by juveniles are treated as acts of "delinquency" in the federal system and are not handled the same way as crimes committed by adults.

Authorities have not said how they think the girls became interested in helping the Islamic State militants.

The announcement comes a month after 19-year-old Shannon Conley of Arvada, Colorado, pleaded guilty to charges that she conspired to help militants in Syria. 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.

Agents, who had been overtly trying to stop Conley, arrested her in April as she boarded a flight she hoped would ultimately get her to Syria. She could face up to five years in federal prison and a $250,000 fine when she is sentenced in January.

Foreign fighters from dozens of nations are pouring into the Middle East to join the Islamic State group and other terrorist organizations. U.S. officials are putting new energy into trying to understand what radicalizes people far removed from the fight, and into trying to prod countries to do a better job of keeping them from joining up.

___

Associated Press writers Eric Tucker in Washington and P. Solomon Banda in Aurora contributed to this report.