Between 400 and 700 Germans are estimated to have joined Isis so far.
Around 100 have died while fighting in Syria and Iraq.
Yet many allegedly begin contributing to the extremist group from within Germany itself - with members using criminal activity to fund terrorism in Middle Eastern countries, according to reports by Der Westen.
Last November, a nationwide raid culminated in the arrest of eight men in North Rhine-Westphalia.
Ranging from 21 to 36 years old, the men have already run up a bill of indictment 435 pages thick – and in Cologne, their case is about to come to trial.
The men are thought to have made around €10,000, the bulk of which would have gone directly to Isis, but which also would have funded the men's travel to the war zones.
The scope of their alleged
The German government estimates Isis's capital stock to be between one and two billion euros, much of which comes from donors.
Yet Isis also gleans an estimated $20,000 a day from oil smuggling, as well as raising funds through hostage-taking, blackmail and taxes of up to 15 percent on all who live in Isis-controlled areas.
Stuttgart gang makes €130,000 contribution to war fund
In Stuttgart, meanwhile, five men currently stand accused by federal prosecutors of providing Islamist group Ahrar al Sham with combat gear worth up to €130,000.
The group allegedly sent 7,500 pairs of boots and 6,000 military jackets through Turkey into Syria.
Criminal archaeologist Michael Müller-Karpe told Der Westen the Isis are also selling ancient artifacts on the German market.
The rise of ISIS spells "a new level of threat to our archaeological heritage," he told Der Westen.
"Millions" of items of cultural value have now found their way onto the market, he believes.
Culture Minister Monika Grütters has also warned that the country must take care not to become an center for illegal trade in cultural artifacts.
However, with the Federal Criminal Police Office stating it has "no knowledge of an Isis-driven trade in such artifacts within Germany," the scale of the problem remains uncertain.
"Grave robbing and the plundering of archaeological and antique finds both add to the income of Isis," the Bundestag reported back in March this year.
"It can be assumed that this trade exists on the European market, as well as in Gulf States and north American nations."