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Eric Gay
Interim Austin police chief Brian Manley, left, talks with FBI Special Agent in Charge Christopher Combs, right, near the site of Sunday's explosion, Monday, March 19, 2018, in Austin, Texas. Fear escalated across Austin on Monday after the fourth bombing this month — this time, a blast that was triggered by a tripwire and demonstrated what police said was a "higher level of sophistication" than the package bombs used in the previous attacks. (AP Photo/Eric Gay)

AUSTIN, Texas — The hunt for the serial bomber who has been leaving deadly explosives in packages on Austin doorsteps took an even 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 directed at someone in particular.

"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 blast happened Sunday night in southwest Austin's quiet Travis Country neighborhood, injuring 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.

The three earlier bombings involved parcels that were left on doorsteps and blew up when they were 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.

Authorities said they are looking at a range of possible motives, including domestic terrorism or that the bombings were hate crimes.

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.

That made it harder to draw conclusions about a possible pattern, further unnerving a city already 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.

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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.

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.