Federal prosecutors said the two men, who were only identified as 25-year-old Rachid B. And 24-year-old Khalid B. in line with German privacy laws, were arrested Tuesday near the western city of Bonn.
Prosecutors alleged both men traveled to Syria in 2013, where they trained in military camps. Rachid B. Initially joined al-Nusra and is accused of kidnapping and holding an alleged spy. He later joined IS and participated in several battles.
The younger brother, Khalid B., joined IS right away, went to a military training camp and also participated in IS battles.