SPIEGEL: Mr. Kahl, are you an avid user of Twitter?

Kahl: Not at all.

SPIEGEL: As head of Germany's foreign intelligence agency BND, can you do without? U.S. President Donald Trump seems to be at risk of triggering a global crisis with just a single tweet.

Kahl: What the American president tweets can also be read elsewhere quickly enough. Plus, the U.S. is not one of our intelligence targets.

SPIEGEL: During the campaign, Trump said several times that torture tools such as waterboarding are acceptable. The BND cooperates closely with U.S. intelligence agencies. Does Trump's position make you uncomfortable?

Kahl: Following the election, Trump's advisors quickly countered much of what he said as a candidate. And regarding torture, Trump said: If my secretary of defense, my secretary of state and my security advisor are against it, then we'll leave things as they are. Currently, we don't have any indications that the legal situation in the United States is going to change.

SPIEGEL: Trump's program is "America First." Is there concern that America's willingness to share information and terror warnings with German agencies will fade?

Kahl: Were "America First" to develop into a dogma in intelligence cooperation, that wouldn't be particularly amusing. Thus far, though, there is no indication that such cooperation is being reduced. I have great faith in the American institutions  and am confident that, in the areas that are relevant to our work, sufficient sense and expertise will be exhibited by the new administration as well.

SPIEGEL: The whistleblowing platform WikiLeaks recently published sensitive information from inside the CIA. Should we be worried that the BND also peers into the private lives of German citizens by way of all manner of hacked devices, such as mobile phones, televisions, cars and even smoke alarms?

Kahl: Germans don't need to be worried about that. We have no interest in intruding on people's private lives. The BND has a clear legal mandate: We must obtain information from abroad that is important for the security of our country. The private lives of German citizens are completely irrelevant.

SPIEGEL: Not necessarily. From 2008 to 2011, the BND listened in on the communications of Hansjörg Haber, head of the European Union monitoring mission in Georgia. He is a German citizen and husband of the state secretary in the Interior Ministry.

Kahl: That doesn't contradict what I just said.

SPIEGEL: True. Only after someone becomes a politician, ambassador or employee of an NGO or a European institution might they end up as an intelligence target on the so-called selector list. You have conducted surveillance on many such people ...

Kahl: ... and have drawn the consequences from such incidents. We now have a new BND law, which defines more precisely the conditions under which we may conduct surveillance and when we cannot. These cases are now not only subject to stricter intelligence rules but also continue to be energetically monitored.

SPIEGEL: Will you continue to conduct surveillance on journalists?

Kahl: We will adhere to the rules that are now law. There are different levels for Germans, Europeans and those journalists who work and operate in non-European countries. If a foreigner in Raqqa claims to be a journalist, we are going to conduct surveillance anyway if he is affiliated with Islamic State.

SPIEGEL: Yet you apparently didn't draw any distinction between such a person and reporters working for the BBC and the New York Times . Where is the boundary?

Kahl: That's not so easy to answer from where we sit. But we have no interest in investigating journalists on the whole. Neither domestically nor abroad. We are searching for information that is relevant to our security and looking for people who are planning evil deeds. It can't be avoided that these people sometimes communicate with others who are less suspicious.

SPIEGEL: What is your assessment of Islamic State's current situation in Syria and Iraq?

Kahl: In contrast to other terror organizations, IS seeks to bring territory under its control. Currently, that is being taken from it; IS is losing territory. But that doesn't mean that it is disappearing. IS will continue to play a role and make itself visible.

SPIEGEL: Is Islamic State moving to other countries, like Libya, for example?

Kahl: We see IS activities in Libya. And we are watching activists in Sub-Saharan Africa who used to be called Boko Haram but who now must be assigned to Islamic State. We are seeing IS in Afghanistan, we are seeing IS on the Sinai Peninsula.

SPIEGEL: Are those distinct cells or is there a larger network?

Kahl: There are repeated efforts to undertake operations on behalf of IS, but it then takes some time before they are recognized by IS headquarters. But that doesn't mean that everything is controlled centrally. Some of the things that have taken place in Germany as well were not ordered by Raqqa or some other command post. The treacherous thing about this terrorist entity is that it can ignite itself largely on its own.

SPIEGEL: There is a theory which holds that the more pressure is placed on IS territory, the greater the risk of attack in Europe becomes. Is that true?

Kahl: I wouldn't formulate it so deterministically. Of course IS will ensure that it remains visible and generates successes for its followers.

SPIEGEL: What is more dangerous, that IS fighters wander from one region to another or that people ignite themselves, as you put it?

Kahl: The dangers exist concurrently. First, there are the returnees, the trained jihadists. They are dangerous if they once led lives in the West and are familiar with it. Second, there are those who come as part of the migrant stream. They might not yet have been given a mission, but they radicalize here. Third are those who have lived here for a long time and have become radicalized. The number of Salafists has increased substantially in recent years to 9,700 -- a large reservoir of people who could become violent at some point ...

SPIEGEL: ... because they become indoctrinated in mosques and training centers. Such sites are financed by Gulf states, with whom Germany has tight relations.

Kahl: We have noticed improvements in this regard. It also makes no sense to combat terrorism in the Middle East when missionaries in Germany are promoting it. That is why several members of the German government have traveled to the Gulf states. That has had visible results back home.

SPIEGEL: Since the NSA revelations from Edward Snowden, we have been talking almost exclusively about technical surveillance methods. Are there still classic spies of the type one meets in John Le Carré novels?

Kahl: They still exist too. We cannot forego human intelligence, the classic surveillance techniques with informants. We even want to expand the technique. But of course I don't like talking much about it.

SPIEGEL: But we should. Currently, one of your informants is being tried in court. Ali R. spent months supplying information about IS from Syria to the BND and other German agencies. Now he is facing a long prison term for membership in a terrorist group. What is your view of BND informants being convicted?

Kahl: Wait until there has been a verdict in the case! Generally speaking, though, we have to make sure that people who provide us information aren't punished for doing so.

SPIEGEL: How can you? A human informant in IS has automatically broken the law because he is a member of IS.

Kahl: You have to make a distinction here between a real membership and a membership that is a kind of cover. As a matter of practicality, however, that is a difficult distinction to make because members are forced to prove their loyalty and courage, which could result in the commission of crimes. It is an extremely difficult situation.

SPIEGEL: Islamic State is allegedly running low on money . Can you confirm this?

Kahl: It won't go bankrupt and lose its ability to act in the foreseeable future. But income from oil production is dropping, we can see that much. So too are the taxes and fees, because each loss of territory means a loss of population that can be extorted.

SPIEGEL: What are the consequences?

Kahl: The fighters are no longer able to arm and equip themselves as well as they used to. At some point, the coffers will be empty.

SPIEGEL: In the fight against IS, Turkey is one of Germany's most important allies. Given those ties, what is your view of the recent friction  between the German government and the regime of Turkish President Recep Tayyip Erdogan?

Kahl: It isn't my job to comment on relations between the German government and foreign governments. From the perspective of the rule of law, there have been developments in Turkey that are anything but reassuring. The question is: will the country remain a partner in the security alliance?

SPIEGEL: At the moment, Erdogan seems intent on worsening his relationship with the West.

Kahl: There are always better and worse phases. Intelligence agencies must also work together with states that don't conform to our constitutional principles. Because of its geographic location, we don't want to destroy these channels.

SPIEGEL: Erdogan has claimed that the cleric Fetullah Gülen was behind the coup attempt last July. More than 100,000 civil servants lost their jobs in the aftermath and thousands have been imprisoned. Was Gülen really behind the coup?

Kahl: Turkey has tried to convince us on a number of different levels. But they haven't yet been successful.