Investigators on Monday March 19, 2018, work at the scene of a bomb explosion on Dawn Song Drive in Austin, Texas, that seriously injured two men Sunday. Two people have now been killed and four wounded in bombings over a span of less than three weeks. ( Jay Janner/Austin American-Statesman via AP)

Investigators on Monday March 19, 2018, work at the scene of a bomb explosion on Dawn Song Drive in Austin, Texas, that seriously injured two men Sunday. Two people have now been killed and four wounded in bombings over a span of less than three weeks. ( Jay Janner/Austin American-Statesman via AP)

AUSTIN, Texas (AP) — 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.

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“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 (91 meters) 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.

The tripwire twist heightened the fear around Austin, a town famous for its cool, hipster attitude.

“It’s creepy,” said Erin Mays, 33. “I’m not a scared person, but this feels very next-door-neighbor kind of stuff.”

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.

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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.

Thad Holt, 76, said he is now watching his steps as he makes his way through a section of town near the latest attack. “I think everybody can now say, ‘Oh, that’s like my neighborhood,’” he said.

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.

Noel Holmes, whose house is about a mile away, was stunned by how loud Sunday’s explosion was.

“It sounded like a very nearby cannon,” Holmes said. “We went out and heard all the sirens, but it was eerie. You didn’t feel like you should be outside at all.”

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.

The PGA’s Dell Technologies Match Play tournament is scheduled to begin in Austin on Wednesday, and dozens of the world’s top golfers were to begin arriving.

“I’m pretty sure the tour has enough security to keep things safe in here. But this is scary what’s happening,” said golfer Jhonattan Vegas, already in town.

Andrew Zimmerman, a 44-year-old coffee shop worker, said the use of a tripwire adds a new level of suspected professionalism and makes it harder to guard against such attacks.

“This makes me sick,” he said.

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Associated Press writer Jim Vertuno contributed to this report.