“We are aware of about 100 [such] cases,” the minister told Bild am Sonntag newspaper.
In total, 700 Germans have traveled to Syria and Iraq to join ISIL militants since 2012. A third of them have returned to Germany, de Maiziere said.
ISIL has occupied large areas in Syria and Iraq in recent years. The militants have reportedly recruited thousands of Westerners. US intelligence estimates the number of foreign fighters in the ISIL ranks to be between 20,000 and 30,000.
Earlier in August, the Islamic State group released their first video in German, calling on Muslims in Austria and Germany to come to ISIL-held territories or wage lone-wolf attacks at home.