The men, who range in age from 17 to 26, came into Europe along the same migrant route refugees are using to escape the brutal Syrian civil war.
These latest arrests have highlighted tensions in Europe over ISIS' ability to exploit these routes to infiltrate Western cities. The latest arrests come after other similar cases were revealed in a recent CNN investigation.
The fear that ISIS will hide among refugees has been voiced by some in the United States as well, particularly as the Obama administration prepares to increase the number of refugees
it takes in next year to 110,000.
"We have no idea who these people are, where they come from -- we have no idea," Republican presidential nominee Donald Trump told a crowd in Iowa on Tuesday, hours before the increase was revealed. "We have no idea what we're getting into."
But how significant is the threat in the US? Let's break it down.
ISIS has used migrant routes to move operatives into Europe
In the aftermath of the Paris attack in November, authorities revealed that two of the attackers -- who blew themselves up outside the Stade de France -- entered Europe through Greece using fake Syrian passports and posing as refugees. They were accompanied by two other alleged ISIS operatives who were held briefly by Greek authorities, released a month later, then re-arrested in the aftermath of the attack, as detailed by CNN
Cases like this -- and the arrests Tuesday in Germany -- emphasize what is a difficult reality facing European law enforcement: ISIS has used migrant routes to ferry operatives into Europe, and some of those operatives may still be in hiding.
"They're not entirely sure who is in their country," Heather Conley, director of the Europe Program at the Washington-based Center for Strategic and International Studies, told CNN. "And so now they're scrambling after the fact to try to establish who (these migrants) are."
Europe faces major security challenge
Since the surge in migrants last summer, Europe has taken steps to improve their security vetting by investing in external border control.
But the realities of geography are against them. Because of their proximity to the Middle East and North Africa, where the majority of the influx originates, European authorities aren't able to screen refugees and asylum seekers until they've arrived on their shores.
"They're already in Europe today, and it makes that extremely difficult to do that screening," said Conley, noting that many of these migrants enter Europe either without travel documents or with forged ones.
"Particularly, there was a sense that if you had Syrian passport documentation you received a more favorable hearing by Europeans," said Conley, since they are seen as refugees rather than economic migrants. "So European officials have discovered a lot of forged documents and passports."
What does this mean for the US?
Again, geography is key.
Syrian refugees applying for resettlement in the US are put through a multi-step screening process before they're able to enter the US, and the process can take up to two years.
They typically apply from Jordan, Lebanon or Turkey, where the US has beefed up vetting personnel in recent months.
They are referred by the United Nations, then undergo interviews and background checks. Several federal agencies, including the State Department, the Department of Homeland Security, the Defense Department, the National Counterterrorism Center and the Federal Bureau of Investigation, are involved in the process, reducing the chance that forged documents escape notice and other signs of fake identities are missed.
"Can mistakes happen?" asked Conley. "Yes. But even the incidents we've seen have been so statistically small that I do not think there's any reason for concern."
But what happens when the US increases the number of refugees it absorbs?
A senior administration official told CNN the Obama administration plans to increase the overall quota for refugees in the 2017 fiscal year from 85,000 to 110,000.
This number includes refugees from conflict zones around the world, and it's not yet clear how many of these slots will be open to Syrian refugees specifically.
After resettling just under 1,900 Syrians in the initial years of the civil war, the US committed to admit 10,000 in FY2016 -- a goal it exceeded last month
The administration was able to make this increase by scaling up its effort to screen applicants in the region, boosting staffing at key processing locations in Jordan, Turkey, Lebanon, Iraq and Egypt, and accelerating the security vetting and interview process.
Nevertheless, State Department officials stand by the rigor of their vetting process, and insist refugees are the most thoroughly screened group of travelers to the US.