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ISIS militants in the ancient Syrian city of Palmyra.
The man, whom Der Spiegel identifies as 27-year-old "Harry S.," is in custody in Germany and cooperating with authorities who are conducting an investigation. He reportedly spent three months in Syria with the terrorist group, which is also known as the Islamic State, ISIL, and Daesh.
Harry said that in the spring, shortly after he got to Syria, ISIS asked him and another extremist from Germany whether they could "imagine perpetrating attacks in Germany." He was reportedly asked again later, and responded that he wasn't prepared to return to Germany to mount an attack.
Harry also claimed that while he was in ISIS-held territory, he "frequently heard people talking about attacks in the West" and that "pretty much every European jihadist was approached with the same questions he had been asked," according to the newspaper.
ISIS reportedly wants "something that happens everywhere at the same time," Harry said.
Harry eventually left Syria after witnessing ISIS' brutal executions in Palmyra, according to Der Spiegel. He reportedly said he couldn't stand the violence any longer. He said he snuck out of ISIS territory and made his way back to Germany.
ISIS' influence has been expanding across Europe, as the group seeks to dispatch people to Western countries to carry out attacks.
Earlier this month, the Institute for the Study of War noted that ISIS is "executing a campaign to terrorize and polarize Europe and that the group has "inspired, resourced, and directed attempted and successful attacks in the United Kingdom, France, Belgium, the Netherlands, Germany, Spain, Bosnia, Kosovo, and Turkey since January 2014."
Authorities have thwarted several alleged attack plots in the UK, France, Germany, and Turkey, according to ISW. France in particular has been on high-alert lately, after ISIS-affiliated attackers killed 130 people and injured hundreds more as they took hostages, detonated suicide vests, and shot dozens across Paris last month.
BuzzFeed reported last year that smugglers admitted to sneaking ISIS operatives into Europe with groups of refugees. The smuggler, using the pseudonym "Hassan," told BuzzFeed that the jihadis posing as migrants were going to Europe to launch attacks in the West.
"They are waiting for their orders," he said. "Just wait. You will see."
In addition to sending those from ISIS territory back into Europe to carry out attacks, ISIS seeks to recruit people who are currently in Europe. Westerners - some of whom might be drawn to the propaganda ISIS distributes online showcasing both its violence and the territory it holds in the Middle East - meet ISIS recruiters online and can be radicalized over the Internet. These people might be encouraged to mount "lone wolf" attacks on targets in their home countries.