Similarities found in 4 Austin bombings, police say

AUSTIN, Texas – Fear mounted across Texas' capital on Monday after the fourth bombing this month -- a blast triggered this time by a nearly invisible tripwire, demonstrating what police called a "higher level of sophistication" than the package bombs used in the previous attacks.

Two men were wounded in the explosion Sunday night as they walked near a residential street. The three earlier bombings since March 2 involved parcels that were left on doorsteps and blew up when moved or opened, killing two people and wounding two others.

"We are clearly dealing with what we believe to be a serial bomber at this point, Police Chief Brian Manley said, citing similarities among the four bombs. He would not elaborate.

The latest blast occurred in a different part of Austin, and both of the wounded are white, while the victims in the earlier attacks were black or Hispanic.

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.

The police chief said investigators have yet to establish a motive.

"Is this terrorism? Is this hate-related?" Manley asked. He said investigators will "have to determine if we see a specific ideology behind this."

For days, police have been warning people not to touch unexpected or suspicious-looking packages, a chilling thought since doorstep deliveries are more common than ever because of the rise of online shopping. With the latest bombing, though, the attacks took on an even more sinister cast.

VIDEO: Austin bomb investigators see similarities, issue warning to community after another explosion

Manley said the tripwire design required a higher level of skill to construct and represents a "significant change": While the earlier bombings appeared targeted, the latest one would have hurt anyone who happened to walking by.

"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.

He told reporters: "We're very concerned that with tripwires a child could be walking down a sidewalk and hit something."

Hundreds of agents from the FBI and the U.S. Bureau of Alcohol, Tobacco and Firearms have joined the investigation, and the reward for information leading to an arrest has climbed to $115,000.

Fred Milanowski, agent in charge of the Houston division of ATF, said the latest explosive device was anchored to a metal yard sign near the head of a hiking trail.

WATCH: Inside the mind of a serial bomber

"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 500-plus leads since the bombings began and that there have been "persons of interest" in the cases, though they have so far not led to much.

Police asked anyone with surveillance cameras at their homes to come forward with the footage on the chance it captured suspicious vehicles or people.

The latest victims -- ages 22 and 23 -- suffered what police called significant injuries and remained hospitalized in stable condition.

The attack happened in the southwestern Austin neighborhood of Travis Country. That is far from the sites of the earlier bombings, which took place east of Interstate 35 and killed a 39-year-old man and a 17-year-old boy and wounded two other people.

 The highway has traditionally been seen as a dividing line between the city's more affluent west side and more heavily minority areas to its east. That gave rise to suspicions from police early on that the attacks might be racially motivated.

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 are set to begin arriving.

Fear spread well beyond the blast site.

"This makes me sick," said Andrew Zimmerman, 44, a coffee shop worker on the west side. He said the use of a tripwire adds a "new level" of suspected professionalism that makes it harder to guard against such attacks.

"That's what scares me a little bit," he added.

UPS and FedEx alone made more than 3.2 billion deliveries to U.S. homes last year, more than double from just 10 years ago, according to the industry research firm ShipMatrix.

Erin Mays, 33, recently moved from Los Angeles to the Austin area. Mays said she routinely gets six to eight packages delivered to her door every week and also uses grocery delivery services.

"Everybody talks about how dangerous L.A. is and Austin is supposed to be safer. It's creepy," Mays said. "I'm not a scared person, but this feels very next-door-neighbor kind of stuff."