AUSTIN — Anxious Texans huddled inside their homes Monday as investigators scrambled for clues at the scene of another mysterious explosion here, the fourth in a string of attacks this month that authorities say are the work of a sophisticated “serial bomber” who has been terrorizing this city with increasingly complex devices.

The latest blast, which injured two men walking through a residential area Sunday night, marked an escalation in both the tactics and skills displayed by the bomber or bombers, police said. While the three previous devices were hidden in packages delivered to homes in residential neighborhoods, the fourth device — anchored to a for-sale sign — was left on the side of the road and was rigged with a tripwire, showing “a higher level of sophistication, a higher level of skill,” said Brian Manley, the interim Austin police chief.

“What we have seen now is a significant change from what appeared to be three very-targeted attacks to what was, last night, an attack that would have hit a random victim that happened to walk by,” Manley said at a news briefing Monday. “So we’ve definitely seen a change in the method.”

Manley said the fourth device had “similarities” to the three bombs that detonated in the Texas capital in the past three weeks, killing two people and injuring two others, one seriously. The explosion Sunday night plunged the city further into a frightening uncertainty that has left residents on edge. Authorities have seemed vexed, completely at a loss to explain who could be setting off the devices — or why — and asking the unknown attacker to communicate with them. They also have offered rewards of up to $115,000 for information in the case.

The first three explosives all hit eastern Austin, affecting areas where black and Hispanic residents live, prompting some to question whether the initial blast on March 2 would have prompted more urgency had it gone off in a more affluent, predominantly white neighborhood — like where the attack Sunday night happened, injuring two white men.

“This is a public threat,” Nelson Linder, president of NAACP’s Austin chapter, said Monday. “Now that the geography has changed, it’s going to widen people’s perspectives. Nobody can take this lightly; we’re all vulnerable.”

Linder added: “Like they tell us in the military, when you walk, look down at where you’re walking.”

Police have declined to describe the bombs in detail, saying only that they appear to be the sophisticated work of a person or people who know what they are doing — and noting that the bombmaker has been able to assemble and deliver the devices without setting them off. After telling residents to remain wary of unexpected or suspicious packages, authorities are urging broader caution.

“We’re even more concerned now that if people see something suspicious, they just stay away from it altogether and contact law enforcement,” said Fred Milanowski, special agent in charge of the Houston division of the Bureau of Alcohol, Tobacco, Firearms and Explosives. “Because if they move that package or if they step on that tripwire, it’s likely to detonate.”

Milanowski said bombs using tripwires are activated when any pressure is applied to the wires, and he said that can include people “tripping over it or picking up the package.”

Investigators have said the attacks might have been motivated by racial bias, since the previous explosions killed two black people and wounded a Hispanic woman, though they acknowledged that this is just one theory. The explosion Sunday injured two white men — one 22, the other 23 — walking through Travis Country, a wealthy neighborhood in southwest Austin, far from the other three attacks.

The men were taken to a hospital with serious but non-life-threatening injuries, and Manley said they were in stable condition Monday.

“With this tripwire, this changes things,” said Christopher Combs, special agent in charge of the FBI’s San Antonio office. “It’s more sophisticated. It’s not targeted to individuals. We’re very concerned that with tripwires, a child could be walking down the sidewalk and hit something.”

Combs said the bureau had brought in more than 350 special agents to help with the investigation, describing it as an “unprecedented response.”

Dan Defenbaugh, a former FBI official who spent decades investigating bomb cases, said the use of a tripwire Sunday “dramatically changes the tactics involved.”

The shift in method — from a bomb targeting an individual or their family to a device that could kill a random passerby — evokes past attacks that created climates of fear, such as the Washington-area sniper shootings that gripped the D.C. region for weeks in 2002 as victims were shot at random. And, like during the Unabomber’s campaign of sending explosives through the mail, the Austin attacks thus far defy explanation and have come without any threat or warning.

“Once a bomb builder makes a device, they usually make it the same way each and every time,” said Defenbaugh, who managed the Oklahoma City bombing investigation. “That’s not happening here. I’m also troubled by the fact that there hasn’t been an extortion demand or communication.”

Defenbaugh praised federal law enforcement agencies for flooding the Austin area with hundreds of agents, but said all those personnel will need to be closely managed to ensure key tips don’t slip through the cracks. In the Oklahoma City bombing case, he noted, investigators gathered 1.8 million hotel records alone. The FBI is sending profilers from Quantico, Va., as well as investigative specialists and emergency response personnel from the surrounding areas.

“It’s a huge, complex effort,” Defenbaugh said, adding that investigators have two things working in their favor so far. First, ATF bomb experts are examining the remnants of the devices, which could offer significant clues to how they were made and by whom. Second, he said, no details of the bombs’ construction have become public, which enables investigators to quickly eliminate any tips involving bombs made using different materials.

“I’m sure it’s on the minds of a lot of law enforcement that they have a serial killer on their hands,” he said.

Many in the neighborhood where the fourth device detonated said they felt removed from the terror that had shaken other parts of town — until Sunday.

“It appears that no one is safe, and I’m very fearful for our community,” said Richard Herrington, 75, who was watching the NCAA men’s basketball tournament when he heard the explosion Sunday night. “It’s very concerning that this person is becoming more sophisticated.”

Ellen Troxclair, an Austin City Council member who represents the district where the Sunday explosion occurred, said she was shocked by seeing the blast there.

“The tripwire definitely instilled some fear into this neighborhood,” she said. “They just want to know what’s going on.”

When the blast detonated Sunday, Eliza May said she heard a sound akin to “when the generators go out, but like five times louder.” May said she was ordered to stay in her home Monday morning and was given permission to work there, but she found herself unable to concentrate.

“This thing is overwhelming,” she said. “My house is a crime scene. I can see the FBI right now — they’re in hazmat suits, walking in a line down the street.”

The explosions have effectively occurred in a ring outside of Austin’s core, which includes downtown, the Texas capitol and the University of Texas. The first two bombs were both deadly: Anthony Stephan House, a 39-year-old construction worker, was killed March 2. Then, 10 days later, Draylen Mason, a 17-year-old high school student, was killed in an explosion that injured his mother.

House and Mason were both related to prominent members of Austin’s African American community, and they had family members who are close, raising fears that their race or familial connections might have played a role in the attacks.

Just hours after Mason was killed, a third bomb went off, seriously injuring Esperanza Herrera, a 75-year-old Hispanic woman who was visiting her mother. But that blast puzzled investigators because it was addressed to a different home and apparently exploded when Herrera was carrying it, according to two people familiar with the case.

Police had initially described the explosion that killed House as an isolated incident, though they changed course 10 days later when the additional bombs detonated. Manley said Monday that police still do not have evidence leading them to a particular suspect, and he reiterated his plea to the public for tips and information.

Austin Mayor Steve Adler said in a telephone interview Monday that “with each additional event, the horrible part is that people are getting hurt.” But, he added, “it also means that law enforcement folks get additional forensic evidence.”

The explosion Sunday went off just hours after the Austin police made a public appeal in the case, increasing the reward for information to $100,000 and addressing the bomber or bombers in particular. Gov. Greg Abbott (R) has also offered another $15,000 for information.

“These events in Austin have garnered worldwide attention,” Manley said during the earlier announcement. “And we assure you that we are listening. We want to understand what brought you to this point, and we want to listen to you.”

Berman, Barrett and Flynn reported from Washington. Julie Tate in Washington contributed to this report, which has been updated.

The scene in Austin after a fourth package explosion injures 2

Investigators on March 19 work at the scene of a bomb explosion on Dawn Song Drive in Austin that seriously injured two men March 18. Two people have been killed and four wounded in bombings over a span of less than three weeks. (Jay Janner/AP)