AILSA CHANG, HOST:
Now we go to Austin, Texas, where authorities say they are no closer to understanding who's behind a series of deadly bombs. The latest explosion happened last night. It varied from the earlier ones in a few ways. For one thing, the two men injured were white. Claire McInerny of member station KUT reports.
CLAIRE MCINERNY, BYLINE: Before last night, three bombs left on doorsteps killed two people and injured two others. The packages were left overnight at people's homes, making law enforcement think they were targeted attacks, potentially racially motivated because the first three were left at the homes of people of color. But Sunday's explosion was different. Police believe the bomb was triggered by a tripwire set up near a road in an upscale neighborhood. The explosions have happened miles apart, but police do think they are related. The change in the suspect's method concerns FBI Special Agent Christopher Combs.
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CHRISTOPHER COMBS: We're concerned about the change in the M.O. It shows an increased knowledge of explosives. We don't know why he's done that. We don't know why he's picking the places that he is. We don't know why he's doing it in general.
MCINERNY: Combs said anyone walking down the street could have triggered the bomb. And the shift from a targeted attack to a random one is confusing.
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COMBS: Is it terrorism? Is it civil rights? Is it something completely off the radar? We don't know that. And that's why it's important to get this dialogue going.
MCINERNY: There are currently 500 officers from around Texas and the U.S. working on the investigation. The FBI even took the unusual step of asking the suspect to contact law enforcement. They want to know the person's motive and what the message is. Today police asked residents of the neighborhood to submit any surveillance video they may have at their homes. Interim Austin Police Chief Brian Manley says he's worried about the shift from targeted package bombs to random explosives.
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BRIAN MANLEY: The suspect or suspects that we are dealing with have a higher level of sophistication than maybe we initially thought based on them changing their methods to a more difficult device.
MCINERNY: Austin police, the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives have released little information about the bombs. The entire neighborhood where the fourth bomb exploded was shut down for most of the day. Residents were ordered to stay inside while police swept the area for evidence and possibly more explosives. For NPR News, I'm Claire McInerny in Austin. Transcript provided by NPR, Copyright NPR.