More than 9,000 suspected terrorist plots against European countries have been discovered in recent years, according to German intelligence services.
Security experts revealed in a report, seen by the Sunday Times, most of the recent cases are linked to migration of late 2015, when Germany opened its borders to Middle Eastern refugees to avoid a humanitarian emergency.
The country registered 1.6 million asylum seekers in total around that time.
Since 2015 police have received more than 1,900 reliable tip-offs about terrorists among the refugee population.
More than 70 investigations are underway, with authorities carrying out an average of three to four house searches a week.
Many of America’s considerable intelligence resources in Germany are focusing on terror threats emanating from migrant communities.
Germany has become Europe’s prime “terror hub”, intelligence sources said.
With ISIS facing defeat in Syria, many more migrants are likely to travel by boat from Libya or Egypt to Europe, according to the intelligence sources whose electronic surveillance tools are trained on the route.
Although the German authorities have evidence that about 25 ISIS terrorists have infiltrated the stream of refugees to attack Europe, the majority of those arrested are young men who became radicalised in Germany after communicating with ISIS handlers over social media.
Tens of thousands of migrants were granted asylum in 2016 without being interviewed by authorities.
More than two-thirds of the 1.6 million refugees in Germany are male and more than half arrived without any form of identification, according to official estimates.
About 230,000 rejected asylum seekers have been marked for deportation, but the lack of documents makes it difficult for the authorities to remove them from the country.
Due to the European Union’s (EU) Shengen zone, migrants who reach German can move freely across the borderless continent. Thousands have disappeared without a trace.
Peter Altmaier, Germany’s refugee co-ordinator and acting finance minister, said the lack of information about people entering and leaving the EU was part of the problem.
Mr Altmaier told Bild: “No one knows exactly how many asylum seekers have left Germany without being registered.”