A group of German nationals formed a "terror cell" which they dubbed "Der harte Kern" (German for "The Hard Core"), the Welt am Sonntag newspaper reported on Sunday.
Authorities arrested 12 men in a series of raids in several German states on Friday.
Germany's Federal Court of Justice, the highest criminal court in the country, ordered the men on Saturday to remain in jail while the investigation continues.
The suspects were accompanied by police to Germany's highest criminal court
While official information remains scarce, local media also reported that the police found weapons while raiding a total of 13 residences across six German states on Friday.