The head of Germany’s domestic intelligence agency says ISIS members slipped into the country along with the more than 1 million refugees who have entered, but added there was no concrete evidence of an imminent attack.  

“We have repeatedly seen that terrorists ... have slipped in camouflaged or disguised as refugees,” Hans-Georg Maassen, the head of the BfV, told ZDF, the German public broadcaster, on Friday. “This is a fact that the security agencies are facing.”

He added: “We are trying to recognize and identify whether there are still more IS fighters or terrorists from IS that have slipped in.”

English translation of the remarks were provided by Deutsche Welle, the German broadcaster.

Maassen’s remarks came a day after police in Berlin arrested three Algerians with suspected ties to ISIS who, they say, were planning a terrorist attack on the German capital. Among those arrested was a 35-year-old man held at the refugee shelter in Attendorn, Germany, who was the alleged head of the cell. Authorities say he entered the European Union posing as a refugee.

“We are in a serious situation and there is a high risk that there could be an attack,” Maassen told ZDF. “But the security agencies, the intelligence services and the police authorities are very alert and our goal is to minimize the risk as best we can.”

Europe has been on high alert since last year’s deadly attacks on Paris that killed 130 people. That attack, which was claimed by ISIS, coincided with Europe’s largest refugee crisis since World War II. Hundreds of thousands of people are fleeing the civil war in Syria and unrest elsewhere and seeking refuge in Europe. Germany, which has announced an open-door policy for Syrian asylum-seekers, is their most-favored destination.

But critics of Germany’s policy, and indeed of Europe’s acceptance of the newcomers, point to the growth of ISIS in Syria and other restive parts of the Arab and Muslim world. Terrorists, they say, could enter Europe posing as refugees. Indeed, at least some of the Paris attackers, all European-born members of ISIS, entered the EU from Turkey using false Syrian passports and posing as refugees.