Two of the girls, sisters aged 15 and 17, had sent fake sick notes to their school in Arapahoe County before stealing $2,000 from their parents and leaving in secret with a 16-year-old relative.
Their parents reported the girls missing on Friday when they realized that the money and their passports were gone.
An FBI spokeswoman told the Denver Post that the agency “assisted with bringing the individuals back to Denver,” adding that they were safe and had been reunited with their families on Tuesday.
Police told the Frankfurter Allgemeine Zeitung that they had "taken the girls into temporary protective custody and organized a voluntary return trip to America the next day" at the request of the parents and the US Consulate.
One of the girls reportedly told police in Frankfurt that she was on her way to study in Turkey.
But an unnamed US official told AP that the evidence gathered so far made it clear that the girls were heading to Syria, although he could not say whether they were in direct contact with fighters there.
Investigators are examining evidence related to the case, including searching the girls' computers.
The two sisters were from a family with roots in Somalia, while their relative was of Sudanese origins.
A spokesman for the Somali family said that the girls were healthy and had been picked up by their family from the Denver International Airport on Monday.
The family was nervous about all the media attention and still had questions of its own about what the girls had done, he added.