ROBERT SIEGEL, HOST:
Investigators are gathering more clues about the man who fatally shot five police officers in Dallas last week and wounded nine others. The chief of police there, David Brown, told reporters today that while it appears the gunman acted alone, a roster of federal and local law enforcement officials are following every lead to ground to make sure that's the case.
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CHIEF DAVID BROWN: Until you show we've exhausted every lead, I'm not going to be convinced that we know everything about what happened. We're going to turn over every rock. We're going to follow every lead till it's exhausted. You know, I want to make sure there's nobody else out there that had something to do with this.
SIEGEL: And part of that process is trying to understand what motivated the gunman. NPR's counterterrorism correspondent Dina Temple-Raston has been following the story and joins us now. And Dina, what is the latest that investigators are finding out about the gunman?
DINA TEMPLE-RASTON, BYLINE: Well, they've been fanning out around the country to talk to people who knew 25-year-old Micah Xavier Johnson. They have his computer. They're looking at his social media accounts, and sources tell NPR that experts in radicalization have been called in to analyze what investigators have gathered so far. And there are some who believe that if you look at the trajectory of Johnson's life, it falls into a familiar pattern of radicalization.
I mean, in this country, particularly recently when you talk about radicalization, it's usually in the context of young Muslims who end up embracing violent groups like al-Qaida or ISIS, but behind the scenes, law enforcement officials have been worrying for some time about young people who are embracing other kinds of radical ideologies like white supremacy or some parts of the sovereign citizen movement, which broadly is against government, and also some strains of the black power movement.
SIEGEL: Well, what are investigators finding in Johnson's life that suggests radical ideas led him to violence?
TEMPLE-RASTON: Well, we should say right away that there are some questions about his mental stability. Officials tell NPR that his journals are often rambling and confusing. At the same time, he was able to negotiate for two hours with police before he was killed. And police said that he was reasonably cogent when he was talking to them.
So one of the things that they're exploring is the effect that recent video showing police shootings, one from Louisiana and the other from Minnesota, had on Johnson's state of mind. The Facebook live video showing the fatal encounter between police and Philando Castile in St. Paul was watched by millions of people, and Johnson mentioned it when he was in that standoff with police. So officials are looking at his writings to find clues on how those videos might have affected him.
SIEGEL: But for - to hear you tell it, they're not convinced that that's what set him off.
TEMPLE-RASTON: Exactly. I mean, there are many things that are in play when something like this happens, but experts on radicalization are now involved. And they see this as part of a larger puzzle. If you think about it in terms of ISIS, ISIS radicalizes a lot of young men by posting emotional content - imagery that shows women and children suffering, for example.
The young men who were just convicted in Minneapolis a couple months ago for trying to join ISIS talked a lot about this. They said they watched ISIS videos showing women and children being killed in Syria, and they felt they needed to do something to help.
SIEGEL: And do police see parallels in Johnson's case?
TEMPLE-RASTON: Well, that's one of the theories they're working on. I mean, clearly something bigger led him to violence. But right now they're trying to understand motive in this context. Law enforcement officials say Johnson had been following various radical black power groups on Facebook for quite some time. They believe he'd attended some of their rallies.
And remember; he was discharged from the military, and his parents say that when he came home, he was at a real loss. So law enforcement behavioralists (ph) are looking at whether he used these extremist groups as a way to feel connected to something bigger than himself. And they're trying to figure out what led him to begin to see himself as essentially a self-appointed warrior for African-Americans. I mean, this is still early days in the investigation, but this is definitely one of the threads that they're pursuing.
SIEGEL: Dina, the Dallas police chief said that they found an arsenal in Johnson's house, suggesting that he had a bigger attack in mind. What more can you tell us about that?
TEMPLE-RASTON: Well, that also fits into this radicalization narrative and one of the reasons why they brought radicalization experts in to look at this. The narrative begins with holding individuals responsibility - responsible for things that appear to be unfair. And then it moves into seeing the whole system as corrupt and violent. And that might go some way toward explaining why he targeted so many police in Dallas on that Thursday night.
SIEGEL: That's NPR counterterrorism correspondent Dina Temple-Raston. Dina, thanks.
TEMPLE-RASTON: You're welcome.
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