Marwan Abou-Taam works with the state criminal police in the southern German state of Rhineland-Palatinate.

DW: When jihadist fighters return from Syria or Iraq to Germany and other European countries, what sort of danger do they pose?

Marwan Abou-Taam: The problem with those who come back is, first of all, that they had already been radicalized to the extent that they went to war. I definitely assume that some of those who went became disappointed and have perhaps even turned away from the idea of jihad. I think a different part of that group is dangerous because we're talking about people who had developed a hatred for the Western way of life when they left. They've come into contact with death - having seen it or even killed personally. They are - in addition to the factors that caused their radicalization - also now traumatized and trained with weapons. Even if they don't have any mission to carry out attacks here, they're dangerous because they can't be controlled. Then, there is a third group of people who have probably come back to Europe with the mission of waging jihad in Western societies. They are the particularly dangerous individuals because they are already planning violence.

Can security officials get the problem under control?

We have a situation that's difficult to get a complete handle on because there are also people who have left to fight in the Middle East without the security agencies even being aware of it. This fourth, clandestine group can work here under the radar. That represents a clear risk for the societies in which they're located.

German intelligence estimates around 450 jihadists have left Germany for Syria. How big might this fourth group be?

The group that we're not aware of is presumably bigger than those whose departure has been tracked. What's problematic with Syria - in comparison with, say, Afghanistan - is that traveling to Syria is relatively simple. When you fly from Germany to Turkey, you don't stand out because so many people travel there. One can't keep track of who is vacationing there and who is heading over to the border region.

Has it in fact occurred that jihadist fighters have returned with specific missions in mind?

We're aware of the French attacker at the Jewish Museum in Brussels. He wasn't just any given fighter in Syria. Instead, we have statements from witnesses saying he was involved in kidnapping French citizens. That means he targeted French interests there. It can be assumed that he returned to Europe having decided to carry out a strike here. In Germany, we had the case of a cell in the Sauerland region whose members had gone toward Afghanistan to take part in jihad. They were given the assignment to come back and undertake an attack in their home country. As such, there are examples of such missions that have been handed to returnees.

Could we see an increase in such attacks if the "Islamic State" (IS) terror organization comes under pressure in Syria and Iraq?

In the short term, definitely. I would even assume that IS can send fighters here strategically to carry out strikes. People should be aware that there's a qualitative difference here with al Qaeda. Al Qaeda fighters were primarily Gulf Arabs who were generally not naturalized European citizens. Consider those behind the September 11 attacks - hardly any of them had a European passport. With those leaving for Syria and Iraq, by contrast, we're dealing foremost with people who are either of European background, or are from immigrant families but have European citizenship. They can come back to Western communities much more easily, especially when their departures weren't even noted - and undertake missions intended to bring terror to the West.

Has Western states' involvement in the battle against IS increased the risk of attacks in Europe?

In the near term, I think that Europe and the Western world's activity in Iraq and Syria is contributing to the danger of attacks. On the other hand, it must be said that the logic of al Qaeda, and its offshoots IS and the al-Nusra Front, already dictates that supporters attack wherever possible. The danger has increased, but it was always there.

Could we see a wave of returns if the tables turn for IS?

In Afghanistan, we've observed that some have come back. Although there we have a situation where we're dealing with wanderers who travel from one conflict region to the next. It can't yet be determined whether Western activity could decisively help to beat back IS, but it's improbable. The US says it will take at least three years to weaken IS. I don't think that a wave of returnees is to be expected.

Marwan Abou-Taam works with the state criminal police in Rhineland-Palatinate, where he is responsible for the department focusing on politically motivated criminality. He has a doctorate in Islamic Studies in which he focused on Islamist terrorism.

Interview: Dennis Stute / gsw