Fear escalated across Austin on Monday after a fourth bombing in a month, 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.

Two men in their 20s were wounded on Sunday night as they walked along a street and were hospitalized in stable condition. The three earlier bombings since 2 March left two people dead and two wounded.

“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, the city police chief, Brian Manley, said.

Investigators have yet to establish a motive, he said, adding: “Is this terrorism? Is this hate-related?”

Investigators will “have determine if we see a specific ideology behind this”, he said.

Manley said Sunday’s blast involved a tripwire, unlike the first three attacks in which package bombs were left on people’s doorsteps. That represents a “significant change” in that the previous bombings appeared targeted while the latest one would have hurt any random person walking by, Manley said.

The device this time entailed “a higher level of sophistication, a higher level of skill”, the chief said.

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 asked anyone with surveillance cameras at their homes to make the footage available in case suspicious vehicles or people could be seen.

Sunday night’s explosion happened in the south-western neighborhood of Travis Country. That is far from the sites of the earlier bombings, which occurred over two-plus weeks in residential neighborhoods east of Interstate 35.

The 22- and 23-year-old men injured this time were white, unlike the victims in the earlier blasts, who were black or Hispanic.

Manley warned people not to touch suspicious bags, boxes or backpacks, especially if they had wires protruding. “We need people paying attention to suspicious objects,” the chief said.

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 be reassured by the massive police response. 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 ended on Monday for the University of Texas and many school districts, meaning people who were out of town have returned home to heightened fears. The university’s campus police warned students to be wary and to tell 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 would not 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 had lived in Austin his entire life.

“This makes me sick,” he 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.

Police said the victims were hospitalized with injuries that were not life-threatening. The latest 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.

The PGA’s Dell Technologies Match Play tournament is scheduled to begin in Austin on Wednesday, with dozens of the world’s top golfers set to begin arriving a day before.