The girls, aged 16 and 17, were intercepted by Romanian authorities en route to Syria, after they were reported missing in Austria by one of their mothers.
When police were investigating their disappearance, they found the mobile phone of the 16-year-old Chechen girl, which showed she had been in contact with Islamist fighters from Isis, and had arranged to marry a jihadist upon arrival.
Prosecutors wanted the girls placed into custody while they investigated whether the girls could be considered to be members of a terrorist organization, but the judge in Salzburg ruled that they had committed no crime, and ordered them to be released.
"In her opinion the girls' behaviour is not criminal, not yet, because they were stopped in Romania and did not really get to join a terrorist organization," said a spokeswoman from the Salzburg court. No travel restrictions were imposed on the girls. Prosecutors have 14 days to appeal against the decision while they continue their investigations.
Federal Minister for Women and Civil Service Gabriele Heinisch-Hosek said that she was "very sorry" that the two girls were willing to join the jihadists, and that she would check to see how the two got in touch and whether school checks for radicalization had identified the girls as vulnerable.