The teenagers, two sisters of Somali descent and a friend whose family is Sudanese, were detained when their flight landed in Frankfurt on Friday after the FBI flagged their passports.
"The girls aged 15, 16 and 17 were stopped at the Frankfurt airport by FBI agents. They have now returned to the US and have been reunited with their families in Denver," FBI spokeswoman Sue Payne said yesterday.
Girls' parents said they had no indication their children planned to travel.