RENEE MONTAGNE, HOST:
The investigation into last week's Brussels attacks has revealed a disturbing development. Authorities say the two brothers at the center of the attacks had been watching a key Belgian nuclear researcher. There's speculation that might have been part of an effort to get material for a dirty bomb. NPR's counterterrorism correspondent Dina Temple-Raston reports from Brussels.
DINA TEMPLE-RASTON, BYLINE: The story begins with a small camera, hidden in the bushes of a home in Belgium, belonging to a prominent nuclear researcher. NPR has withheld his name to protect his safety. Belgian and U.S. officials say that last summer two men were seen outside the researcher's home. A passenger got out of the car and retrieved something from under a hedge. And then they disappeared. Who the two men were and what they were doing was a mystery until...
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UNIDENTIFIED REPORTER #1: Two brothers who carried out the deadly suicide bombings at the airport and metro station in Brussels.
UNIDENTIFIED REPORTER #2: (Through interpreter) Identified Khalid el-Bakraoui as the suicide bomber at Malbec metro station. They say his brother, Brahim, blew himself up at the airport.
TEMPLE-RASTON: The Belgian attacks helped authorities figure out what they'd seen last summer. Investigators are now certain the two men they saw that night were the el-Bakraoui brothers. And what they retrieved from under the bushes was a video camera trained on the researcher and his family. Twelve hours of this video was discovered last fall in an ISIS safe house after the attacks in Paris. Belgian authorities believe the brothers may have been plotting a kidnapping. Matthew Bunn is a specialist in nuclear security at Harvard's Kennedy School.
MATTHEW BUNN: This is a pretty common terrorist or criminal tactic - to kidnap a family member to coerce someone to help the terrorists.
TEMPLE-RASTON: And what the terrorists may have wanted was radiological material.
BUNN: If what they wanted was to get radiological material for a dirty bomb from the nuclear site, they were a little befuddled because it's much easier to get that from a hospital or an industrial site.
TEMPLE-RASTON: They aren't the only ones who are confused. Dirty bombs are often mistaken for suitcase-nukes. Remember season two of the television show "24?"
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UNIDENTIFIED ACTOR: (As character) There's a nuclear device under terrorist control.
TEMPLE-RASTON: A dirty bomb is a regular bomb in most every way. Except, explosives are typically wrapped around radioactive material. Steve Sin focuses on illicit trafficking of those kinds of materials at the Terrorism Center at the University of Maryland. He says the effects of a dirty bomb really depend on how it's made. If you have a powdery, radioactive substance, like cesium, and you wrap that in an explosive, it does more damage because it disperses more widely.
STEVE SIN: So imagine if you have baby powder on your counter, and you blow on it, and it goes everywhere.
TEMPLE-RASTON: But if you have something like cobalt-60, which is easier to obtain, it's usually in solid form and is less effective.
SIN: So if you have cobalt-60 as your material and you blow it up, what you end up with is smaller chunks within the certain radius, and it contaminates that particular area. And it doesn't have fallout or anything like that because it's not in the air.
TEMPLE-RASTON: It doesn't have fallout, he says, because it isn't in the air. If last week's explosions had been dirty bombs, the casualty numbers probably would've been about the same. But the cleanup - much worse. Brecht Volders is a Ph.D. candidate at the University of Antwerp in Belgium. Volders says if ISIS had detonated dirty bombs last week, that would've been a turning point for terrorism.
BRECHT VOLDERS: The psychological impact can be very strong, as they would be the first group to use it. Terrorism is always about creating a sense of fear.
TEMPLE-RASTON: Even though nothing came out of the surveillance video, Belgian and American officials say the landscape has changed. The idea of a terrorist group trying to secure materials for a dirty bomb used to be theoretical. Now it's specific. Dina Temple-Raston, NPR News, Brussels.
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