Since the most recent wave of refugees started arriving in Germany the Federal Criminal Police Office (BKA) has received 369 tip-offs about potential suspects, the Neue Osnabrücke Zeitung (NOZ) reports.
Forty of these tip-offs have become more concrete, prompting investigations of some individuals on suspicion of belonging to a terrorist organization and preparing an act of terrorism.
“More terror attacks cannot be ruled out,” a spokesperson for the BKA told NOZ.
But the Interior Ministry cautioned that many of the tip-offs had no substance behind them, saying that many were made by other asylum seekers as an act of revenge against an individual or because of rumours spread through camps.
At other times people with right-wing extremist beliefs have given tip offs to try and discredit asylum seekers, Ministry spokespeople said.
Wolfgang Bosbach, an MP for Angela Merkel's Christian Democratic Union (CDU) warned that there are massive problems with the registration of refugees when they arrive in Germany, and that it is often impossible to identify them.
In recent months 60 percent of new arrivals have reached Germany without passports or identity papers, he said.
“The dangers that arise from this must be taken very very seriously - that is something that the attacks in Brussels and Paris have made clear to us,” said Bosbach.
“Investigators need to check every single tip-off about jihadis smuggled in among the refugees with the utmost of diligence,” the CDU politician added.
But the Left Party, Germany's official opposition in the federal parliament, warned against isolated cases leading to a prejudice against refugees in general.
“The possibility that among the large number of refugees there are also Isis members - perhaps even trained attackers - should not lead us to put every refugee from Iraq and Syria under general suspicion,” Left Party interior affairs spokeswoman Ulla Jelpke cautioned.