German authorities continue to gather more clues about Ali David S., the Iranian-German teenager who killed 9 people and wounded dozens in a shooting rampage in Munich on Friday, before taking his own life.

A search of the suspect's home found the German version of "Why Kids Kill: Inside the Minds of School Shooters," a 2010 book by US psychologist Peter Langman on the psychological drivers of school shootings.

Investigators also said Ali David S. conducted research on the far-right terrorist Anders Breivik, had visited the site of a 2009 school shooting in Germany, regularly played a violent first-person shooter video game and had mental health issues.

DW spoke with Langman over the phone to gain a better grasp of why the teenager might have gone on a shooting rampage and the reason he may have had Langman's book. The interview has been edited for clarity.

DW: What do you make of the fact that the shooter had a copy of your book?

Peter Langman: It's really not that surprising in some ways - it's surprising to me that it was my particular book. School shooters often do a lot of research, they go online to study other incidents, find other shooters they admire and read about other shooters.

There may be several purposes for researching. They may feel like they are abnormal or a misfit. Reading about other people who had the same thoughts might give them a peer group in their own mind, and they might feel less abnormal. They might get a sense of social approval that what they are doing isn't so bad, that other people have done this and provide reasons for doing it.

You have said there is no "one profile" that fits any shooter, but you break it down into three general categories: psychopathic, psychotic and traumatized. Which one would you say the shooter is based on what we know?

We don't know enough yet to know his typology. It often takes a long time for psychological information to come out, and sometimes there is not enough information to make a determination.

At this point, he doesn't sound like a psychopathic shooter. Those are typically extremely narcissistic people. They also have good social skills. They can even be charming and charismatic. They are often much more socially successful than the Munich shooter. I'm not seeing these traits; he seems more shy, extremely insecure and anxious.

Investigators have said the Munich shooter enjoyed playing first-shooter video games similar to other mass shooters. This finding has sparked debate over violent computer games and their role in shootings. Is it common for shooters to play such games?

It's a complex issue because most of the millions of teenagers who play those video games never kill anybody. So there is no simple connection between playing those games and going on a rampage.

However, when you look at a good number of school shooters, they often not only played those games, but in some cases were really obsessed. That was the kind of mental world they lived in. That can serve a couple of purposes: it desensitizes people and it's a way to rehearse, to practice pulling a trigger on a target.

What about the role of suicide? Many of these types of shootings end with a suicide or the shooter going into the rampage knowing they will likely be killed.

Based on a sample of 48 shooters in my most recent book, about half intended to die in their attacks. Some people tend to assume school shooters and mass shooters are suicidal, but it's only about half the time.

Many of these shooters are both suicidal and homicidal. They want to die, but they don't want to die alone and in an insignificant way. They want to make other people pay for the suffering and get revenge in some way. Sometimes they also want the fame and recognition that goes with this kind of act so that they are not dying alone -- unknown and insignificant. They are trying to give their lives some sort of larger meaning, and the best way they can do that, unfortunately, is through violence.

You mentioned shooters do their research about other shootings. Such shootings are less common in Europe and Germany compared to the United States. Do you see a phenomenon of copycat shootings sort of jumping over the Atlantic?

I definitely think so. If you go back to Robert Steinhauser (a 19-year-old who killed 16 and wounded seven in a 2002 school attack in Erfurt, Germany) or Tim Kretschmer (a 17-year-old who killed 15 and wounded nine in a 2009 school attack in Winnenden, Germany), they were very aware of the shootings in the United States, especially the Columbine shooting. [Investigators say the Munich shooter visited the site of the Winnenden attack, the ed.].

You classify attacks into four types: random, targeted individuals, targeted groups and mixed. How would you classify this attack?

It seems to be random in that there was no specific individual he appeared to be going after.

Some, however, have suggested that he was specifically shooting young people. I don't know if most of the victims were teenagers because that was who happened to be in the vicinity or if he was singling out the teenagers from among a crowd.

Even though victims were random in that he didn't know then, it's possible he was going after his peers. If that is the case, then there are a couple ways we might interpret that. He apparently felt victimized and was victimized by his peers, but he didn't go after the specific kids who had beaten him up.

He may have had a generalized rage against his peer group. Another possibility that gets much less attention is the idea of envy. A lot of times shooters feel abnormal. They are depressed. They aren't socially successful. They look around at the world and see everyone is happy and successful, and there is profound envy. That envy turns to hatred because it doesn't seem fair, so the shooters go after the people they most envy.

German prosecutors have said the shooter did extensive research into far-right terrorist Anders Breivik, who killed 77 people in Norway exactly five years to the date of Friday's Munich shooting. What do you make of this?

I don't know what to make of reports that the shooter expressed some anti-foreigner rhetoric, whether those reports are true because his parents are Iranian immigrants. In the wake of an attack like this there is a lot of misinformation.

Breivik's mission was to get the Muslims out of Europe and to save white European civilization. So I don't know why the Munich shooter would be drawn to that given that his family came from Iran - I don't know if he was Muslim. It would be very strange for him to find an idol in Breivik unless he just admired his ability to kill a lot of people and had no interest in the political ideology of it.

How can such rampage shootings be prevented?

There is a lot we know about the warning signs. It's a matter of educating the public so they recognize a warning sign and know what to do when they see it. Often there is a long trail of warning signs: people talk about what they are going to do; sometimes they warn friends to stay away so they don't get hurt; sometimes they invite friends to join attacks; they may make threats to people they intend to kill; they may post things online; and sometimes they hand in school assignments expressing a desire to kill somebody.

Sometimes the warning signs are very obvious, but people don't recognize them or don't take them seriously. Or they are afraid to report it for various reasons. Certainly, there are a lot of spoiled attacks that we don't hear about because it doesn't get media attention if it doesn't happen.

To see more on prevention, visit Dr. Langman's website section on warning signs of mass shootings: https://schoolshooters.info/prevention

Peter Langman is an expert on the psychology of school shooters. He is the author of two books on the topic, "School Shooters: Understanding High School, College and Adult Perpetrators" and "Why Kids Kill: Inside the Minds of School Shooters."