AUSTIN, Texas - The hunt for the serial bomber who has been leaving deadly explosives in packages on Austin doorsteps took a new, more sinister turn Monday when investigators said the fourth and latest blast was triggered along a street by a nearly invisible tripwire.
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. Underscoring that point, a relative says the most-recent explosion left what appeared to be nails stuck in his grandson's knees.
"The game went up a little bit -- well, it went up a lot yesterday with the tripwire," Christopher Combs, FBI agent in charge of the bureau's San Antonio division, said in an interview.
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."
"It was so dark they couldn't tell and they tripped," Grote said. "They didn't see it. It was a wire. And it blew up."
Grote said his son, who lives about 100 yards away from the blast, heard the explosion and raced outside.
"Both of them were kind of bleeding profusely," Grote said.
That was a departure from the three earlier bombings, which involved parcels left on doorsteps that detonated when moved or opened.
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.
"We are clearly dealing with what we believe to be a serial bomber at this point," Austin Police Chief Brian Manley said, citing similarities among the four bombs. He would not elaborate, though, saying he didn't want to undermine the investigation.
While the first three bombings all occurred east of Interstate 35, a section of town that tends to be more heavily minority and less affluent, Sunday's was west of the highway. Also, both victims this time are white, while those killed or wounded in the earlier attacks were black or Hispanic.
Those differences made it harder to draw conclusions about a possible pattern, further unnerving a city on edge.
Fred Milanowski, agent in charge of the Houston division of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, said the latest bomb was anchored to a metal yard sign near the head of a hiking trail.
"It was a thin wire or filament, kind of like fishing line," he said. "It would have been very difficult for someone to see."
Milanowski said authorities have checked over 500 leads. Police asked anyone with surveillance cameras at their homes to come forward with the footage on the chance it captured suspicious vehicles or people.
Stacey Rosas lives in the neighborhood rocked by Sunday night’s explosion. She and her husband tried to stay calm for their two young children.
“I run this neighborhood every day. Yesterday, I literally ran here at 3:00 p.m.,” she said. “It was really unsettling to me because a lot of times I have my kids in the stroller or my daughter is on a bike. It was just a few hours before the incident. I'm angry. I'm scared. It hits a little too close to home.”
Rosas says she and her husband told their 7-year-old not to go near anything with wires or that looks odd.
“I'm angry my 7-year-old knows what a trip wire is, and we had to have this conversation,” Rosas said. “And it is sad to me that this is our world right now."
Chief Manley had no new leads when he spoke to reporters Monday evening.
“We so often get locked into our own lives into our own technology, we don’t connect with our community and our neighbors,” he said. “And I think that’s a really important message here is we have to work together here in Austin to solve what’s happening here. But I think it goes beyond the borders of Austin.”
Spring break ended Monday for the University of Texas and many area school districts. University police warned returning students to be alert and to tell their classmates about the danger, saying, "We must look out for one another." None of the four attacks happened close to the campus near the heart of Austin.
Gov. Greg Abbott has approved $265,000 in emergency funding for the Austin Police Department to purchase portable x-ray machines. Authorities have received more than 700 calls about suspicious packages since the first explosion.