Three teenage girls from Denver allegedly had plans to join Islamic State (ISIS) militants in Syria after stealing their parents' money and flying to Germany, reports CNN.

The girls may have succeeded had it not been for a phone call from the Denver area school the daughter of Assad Ibrahim attended informing him that she had not arrived to class on Friday.

Ibrahim called her cell phone. She answered, and according to the Arapahoe County Sheriff's Office who opened a missing person case, she told him that she was late to class.

The American-Sudanese girl said nothing about being with her two friends of Somali descent.

It wasn't until the sisters' father discovered $2,000 and their passports missing that the teens' parents suspected something.

The two families notified the FBI, suspecting the girls were traveling to Turkey.

The two sisters, ages 15 and 17, and their friend flew from Denver to Frankfurt, Germany over the weekend. They were stopped at the Frankfurt airport by FBI agents who detained and questioned them for an entire day before returning the girls to Colorado, according to a FBI spokesperson.

The families reported no previous issues with the girls.

Deputies closed the missing person's case Monday after being notified of the girls return.

It is unknown whether the girls will face criminal charges. Juvenile crimes are treated as acts of "delinquency."

This is not the first time American women have attempted to wage jihad. One month ago Shannon Conley, 19, of Arvada, Colo. plead guilty to charges that she conspired to join militants in Syria. She told agents her goal was to marry an online suitor who claimed to be a Tunisian man fighting with ISIS in Syria.

Conley had been arrested in April as she boarded a flight to Syria. She faces up to five years in federal prison and a $250,000 fine. She is scheduled to be sentenced in January.