Investigators are trying to discover 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 US 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 heading for Syria, though the official said the authorities 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. Colorado has a growing Somali community drawn by jobs in slaughterhouses.

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

The official said the girls were headed toward Turkey en route to Syria, and investigators were reviewing evidence, including information on the girls' computers.

Another US official called the case "concerning" to both the community and the country in general.

The officials spoke on condition of anonymity because they were not authorized to discuss the investigation.

The girls' fathers reported them missing after they skipped school on 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 at Frankfurt airport for an entire day before being detained, questioned and returned to Denver, where they were further questioned by the FBI before being sent home.

They told authorities they had gone to Germany for family reasons, but would not elaborate.