German detectives searched the flats of two men in an anti-terrorist raid in the western German city of Mainz on Sunday.
The pair are suspected of belonging to a terrorist organization in the Syrian civil war, a spokesman for the Federal Prosecutor's Office said, adding that there was no concrete evidence that they were planning an attack in Germany.
The investigations aimed to reveal which organization the two men had belonged to, the spokesman said.
The online edition of Der Spiegel news magazine reported that one of the men was a suspected former commander in the Islamic State militia. The 32-year-old has been in Germany since the autumn.
He denies ever belonging to Islamic State, Spiegel reported.