Austin, Texas • Fear escalated across Texas' capital city on Monday after the fourth bombing this month — a blast that was triggered this time by a tripwire, demonstrating what police called a "higher level of sophistication" than the package bombs used in the previous attacks.
Two men were wounded by the explosion Sunday night as they walked along a street and remained hospitalized in stable condition. The three earlier bombings since March 2 involved parcels left on doorsteps and killed two people while wounding two others.
"We are clearly dealing with what we believe to be a serial bomber at this point, based on the similarities between now what is the fourth device" and the previous ones, Police Chief Brian Manley said.
He 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."
The police chief said the tripwire meant it was a more sophisticated device requiring a higher level of skill to construct than the previous bombs. He said the bombing represents a "significant change," in that the earlier bombings appeared targeted, while the latest one would have hurt any random person walking by.
For days, police have been warning people not to touch unexpected or suspicious-looking packages, a chilling thought at a time when people get more doorstep deliveries than ever before because of the rise of online shopping. With the latest bombing, the attacks took on an even more sinister cast.
"With this tripwire, this changes things. It's more sophisticated. It's not targeted to individuals," said FBI agent Chris Combs, in charge of the bureau's San Antonio division."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 US. 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 in a residential neighborhood.
"It was dark out. It was a thin wire or filament, kind of like fishing line," Milanowski said. "It would have been very difficult for someone to see."
Milanowski said that investigators have investigated over 500 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.
Authorities canvassed the area in search of anything suspicious, and residents were warned to remain indoors and to call 911 if they needed to leave their homes.
Manley also asked anyone with surveillance cameras at their homes to make the footage available in case suspicious vehicles or people could be seen.
The latest attack happened in the southwestern Austin neighborhood of Travis Country. That is far from the sites of the earlier bombings, which took place in residential neighborhoods 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.
Police at first suggested the bombings could be hate crimes since the first three bombs involved victims who were black or Hispanic, but the latest attack may have undermined that theory. The 22- and 23-year-old men wounded this time are white.
The police chief warned people not to touch suspicious bags, boxes or backpacks, especially if they had wires protruding.
Mayor Steve Adler said the latest explosion raised anxieties spreading across the city.
"That concern is legitimate and real," Adler said, adding that residents should also be reassured by the massive police response to the attacks. Hundreds of federal agents are investigating, along with Austin police.
"That anxiousness is going to continue until we can find the answer," Adler said.
Spring break ends Monday for the University of Texas and many area school districts, meaning people who were out of town have returned home to heightened fears.
University police warned returning students to be wary 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.
Concern spread well past the immediate blast site.
"This makes me sick," said Andrew Zimmerman, 44, a coffee shop worker on the city's 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.
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 the day before.
Associated Press writer Jim Vertuno contributed to this report.