Germany’s domestic intelligence service has officially confirmed analysts’ worst fears: At least 17 Islamic State fighters have managed to sneak into Europe disguised as refugees.

According to German domestic intelligence service BfV president Hans-Georg Maassen, the vast majority of those 17 militants are either dead or in prison, NBC News reports.

Maasssen also said an attack like the one perpetrated against the Istanbul Atatürk Airport in Turkey was definitely possible in Germany. Maassen said Germany’s biggest problem right now is “Islamist terrorism.” As a result of the attack in Istanbul, authorities have detained two suspected ISIS operatives after three militants killed 44 people.

BfV’s annual intelligence report, released just last week, stated, “it has to be assumed that ISIS is planning further attacks in Europe, including Germany.” That report also noted, “Islamism in Germany enjoys undiminished popularity” and further added that Islamist organizations have aggressively tried to recruit refugees under cover of humanitarian aid.

The agency has kept tabs on more than 230 attempts by radical Islamists to contact refugees.

Other notable officials have delivered similar remarks, and the idea that ISIS fighters have posed as refugees to enter Europe is nothing particularly new. In March, then-Supreme Allied Commander Europe of NATO Allied Command Operations Air Force Gen. Philip Breedlove stated that ISIS was taking advantage of the Syrian refugee crisis to smuggle its own fighters deep into Europe, as well as the United States.

Breedlove said around 1,500 ISIS fighters had returned to Europe from the battlefield. In his same speech to the Senate Committee on Armed Services, Breedlove said that ISIS’ ideology is “spreading like a cancer” among existing refugees.

A poll conducted in November, 2014, found that 13 percent of Syrian refugees said their view of ISIS was either “positive” or “positive to some extent.”

In 2015, over a million refugees streamed into Germany alone.

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