INVESTIGATORS said on Monday, that the fourth and latest blast in Austin was triggered along a street by a nearly invisible tripwire.
A serial bomber has been leaving deadly explosives in packages on doorsteps in the city.
Police and federal agents said that suggests a “higher level of sophistication” than they have seen before, and means the carnage is now random, rather than targeted at someone in particular.
Two people have now been killed and four wounded in bombings over a span of less than three weeks.
The latest happened Sunday night in southwest Austin’s quiet Travis Country neighborhood, wounding two men in their 20s who were walking in the dark. They suffered what police said were significant injuries and remained hospitalized in stable condition.
Police haven’t identified the victims, but William Grote told The Associated Press that his grandson was one of them, saying he is cognizant but still in a lot of pain. Grote said one of them was riding a bike in the street and the other was on a sidewalk when they crossed a tripwire that he said knocked “them both off their feet.”
That was a departure from the three earlier bombings, which involved parcels left on doorsteps that detonated when moved or opened.
The tripwire twist heightened the fear around Austin, a town famous for its cool, hipster attitude.
Authorities repeated prior warnings about not touching unexpected packages and also issued new ones to be wary of any stray object left in public, especially one with wires protruding.
“We’re very concerned that with tripwires, a child could be walking down a sidewalk and hit something,” Combs said.
Investigators are looking at a variety of possible motives, including domestic terrorism or a hate crime. Local and state police and hundreds of federal agents are investigating, and the reward for information leading to an arrest has climbed to $115,000.