Fear escalated across Austin Monday after the fourth bombing this month — this time, a blast triggered by a tripwire that demonstrated what police said was a "higher level of sophistication" than the package bombs used in the previous attacks.
"We are clearly dealing with what we expect to be a serial bomber," Police Chief Brian Manley said at a Monday morning news conference. "As we look at this individual and the pattern and what we're looking at here, we will have to determine if we see a specific ideology behind this."
Two men in their 20s were wounded Sunday night as they walked along a street in the Travis Country neighborhood on the southwest side of the city near the intersection of MOPAC and U.S. Highway 290. The location of the fourth bomb was far from the previous three that were all in residential areas in the eastern part of the city.
Authorities said the bomb was anchored to a metal yard sign near a hiking trail and equipped with a fishing line-thin tripwire. Frederick Milanowski, the special agent in charge for the Bureau of Alcohol, Tobacco and Firearms, said Monday that the wire or filament that detonated the bomb Sunday night would have been very difficult to see.
The men hurt in Sunday's blast were hospitalized in stable condition with injuries that were not life threatening. The three earlier bombings since March 2 left two people dead and two wounded.
The first was a package bomb that exploded at a northeast Austin home on March 2, killing a 39-year-old man. Two more package bombs then exploded farther south on March 12, killing a 17-year-old, wounding his mother and injuring a 75-year-old woman.
Manley said Monday on ABC's "Good Morning America" that both men injured in Sunday night's explosion are white, unlike the victims in the three earlier attacks, who were black or Hispanic. He said the latest device showed "a different level of skill" from the prior ones.
Manley said that the fourth bomb showed "similarities" with the package bombs used in the three prior attacks, but this one was triggered by a tripwire.
He said the motive behind the bombings remains unclear, and asked the person or people behind the bombing to reach out to the police to let them know why they're setting off the explosives.
"Is this terrorism? Is this hate-related?" Manley asked. He said investigators will "have determine if we see a specific ideology behind this."
Manley repeated previously issued warnings for residents not to touch any unexpected packages left at their homes, especially if they had wires protruding.
"We need people paying attention to suspicious objects," the chief said.
While investigators canvassed the area in search of anything suspicious, Manley asked anyone with surveillance cameras at their homes to make the footage available in case suspicious vehicles or people could be seen.
Sunday night, Manley urged people within half a mile of the blast site to stay in their homes and said authorities would keep the surrounding area blocked off at least until daybreak Monday "given the darkness and size of the area that we want to go in and check."
"We want to put out the message that we've been putting out and that is, not only do not touch any packages or anything that looks like a package, do not even go near it at this time," Manley said. Because "we have not had an opportunity to look at this blast site to really determine what has happened."
Manley said Monday officers have completed a sweep of the Travis Country neighborhood and deemed it safe, but he asked those who live there to remain indoors until 2 p.m. He added investigators found nothing else suspicious in the neighborhood during their search.
"It is important right now for anyone in the neighborhood behind us to remain inside and give us time to work through this," he said, adding that any witnesses should call 911 and report what they saw.
Police said the three previous bombs were likely related and involved packages that had not been mailed or delivered by private carrier but left overnight on doorsteps. Manley originally suggested they could have been hate crimes since all the victims were black or Hispanic, but now said that investigators aren't ruling out any possible motive since Sunday's trip wire bomb could have hurt any random person walking by.
On Sunday, police blocked entrances to the neighborhood where the latest blast occurred and put up yellow tape about half a mile from the home where it happened.
Despite the order for those living nearby to stay in their homes, neighbors milled around just outside the tape. Some reported hearing loud booms but couldn't provide many details.
NBC News reports the FBI has sent 350 special agents to Austin as well as extra bomb squads.
"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."
Sunday's explosion came hours after authorities raised the reward by $50,000 for information leading to the arrest of whoever is responsible for the first three explosions. It now totals $115,000.
Sunday was the final day of the South By Southwest music festival, which draws hundreds of thousands to Austin every March. It was also the end of spring break for many area school districts, meaning families who were out of town in recent days are returning to a city increasingly on edge.
The explosions occurred far from the main South By Southwest activities, though a downtown concert by hip-hop band The Roots was canceled Saturday night after a bomb threat. Authorities later arrested a 26-year-old man, and the incident did not appear to be related to any previous explosions.
The university's campus 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.
Austin's school district announced that buses wouldn't be going into the Travis Country neighborhood and that any "tardies or absences due to this situation will be excused."
But concern spread well past the immediate blast site.
Andrew Zimmerman, 44, a coffee shop worker on the city's west side, said he's lived in Austin his entire life.
"This makes me sick," Zimmerman said, noting 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.
Austin Mayor Steve Adler said the latest explosion only further raised anxieties in 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.
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.