German police have arrested two men, one an ethnic Turk, on suspicions that they are members of the Islamic State of Iraq and the Levant (ISIL), federal prosecutors said on Thursday.

However, the prosecutors said there was no evidence pointing to concrete plans or preparations of theirs to carry out an attack.

The two men, identified by the prosecutor’s office in Karlsruhe only as 26-year-old Mustafa C. and 27-year-old Sebastian B., were arrested in North Rhine-Westphalia state.

Authorities are worried about possible attacks in Germany by extremists who have returned to the country after joining ISIL and other similar groups in Syria and Iraq. Fears have heightened after this month’s attacks in France on satire magazine Charlie Hebdo, the police and a kosher market.

Security services say about 600 German residents have joined militant groups in Syria and Iraq. Of those, about 60 have been killed and around 180 are thought to have returned home.

The two suspects are accused of traveling to Syria via Turkey in 2013 and joining a fighting group “Muhajirun Halab” (Aleppo Migrants) which belonged to a group that later joined ISIL.

They are also suspected of having trained to fight a holy war and to have undertaken logistical tasks such as transport for food and supplies to the front lines there.