Early-morning police raids have taken place on eight properties in Berlin.
A mosque association was among the buildings stormed. German authorities say they believe suspected Islamists with links to Syria were using the properties.
Spokesman Stefan Redlich said police suspected the association of recruiting people to fight for an extremist group in the war-torn country.
“We are looking for evidence here, especially training material, but also any other paperwork that might give us an idea as to whether and how many people were recruited. The people are sent down there, trained in carrying out violent attacks and they experience violent attacks. And they pose a potential danger when they return.”
Redlich said there was no concrete evidence of a planned attack in Germany.
No arrests have been made, although police are reportedly investigating a 51-year-old Moroccan national and a 19-year-old Macedonian, who is thought to be in Syria already.