The latest blast was triggered by an invisible wire, according to authorities.
Now fear is growing about another bomb and where it might be, as local law enforcement is stretched thin.
Houston police are stepping in to help.
"It could become overwhelming very quickly, rightfully so, people are being cautious and the call volume is going up," Houston Police Chief Art Acevedo said at a Monday press conference. "I think it's incumbent upon us in law enforcement and government to help one another."
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HPD sent two bomb techs and a K-9 to help with the increased calls for suspicious packages. Meanwhile the investigation is heating up.
"I suspect have a lot more answers then we all know right now. They're crunching this thing, they're looking at all the variables, you're doing communication analysis and spatial analysis in looking at denominators between the victims, and so forth and so on," said former FBI agent Jim Conway.
Conway says this latest bomb was more sophisticated than the other three.
The profile of a "bomber" in today's time has changed, and it may not be who we think.
"Initially people going to think this is a pro, maybe was former military and have some kind of bomb technical experience, but on the other hand, there's so much available and open source today, there is so much available in cyberspace, been so much available, and on the Internet that someone with limited amount of knowledge, unlimited amount of experience, you could put together a device like this," Conway said.
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