Another explosion rocked Austin on Sunday night when two men in their twenties detonated what police say was a tripwire connected to a “suspicious package” left on the side of the road.

Police Chief Brian Manley said early Monday that authorities are “working under the belief” that the latest bomb—which hospitalized the two men—is connected to three other package explosions this month, which have kept the Texas capital increasingly on edge.

“It is very possible that this device was activated by someone either handling, kicking, or coming in contact with a tripwire,” said Manley. “We now need the community to have an extra level of vigilance.”

Manley said in a late-morning press conference that motive remains unclear, but “we are dealing with what we expect to be a serial bomber at this point.”

The new explosion, he said, “shows a higher level of sophistication, a higher level of skill.”

Police notably stayed away from the phrase “domestic terrorism,” stating that they were hesitant to use the term until motive and ideology have been determined.

“With this tripwire, it changes things,” said FBI agent Christopher Combs. “With the trip wire, you really get into random attacks. It easily could’ve been two 8-year-old children walking down that sidewalk that tripped it.”

ATF agent Fred Milanowski noted during the press conference that “any pressure put on that wire” from tripping on it or picking it up could have detonated the explosion on Sunday. This can include, in the future, “a bag, a suitcase, a box,” he said.

Milanowski said the suspect may have brought in some type of yard sign to anchor the tripwire.

Local media reported the wounded men had nails in their legs, but Manley would not confirm that fact, only stating that “these explosions are using different types of projectiles.”

Manley said the two victims were in a residential neighborhood in southwest Austin at about 8:30 p.m. when the explosive went off. Neighbors in the area told reporters the explosion caused “a huge boom” and sounded like an electrical transformer blowing up—“but it was five times magnified.”

Authorities have said residents within the Travis Country neighborhood should remain in their homes until 2 p.m., when the scene is “processed” and “properly cleared of any hazards.” Children in the area have been excused from school by the Austin Independent School District and several private schools.

Officials were searching nearby pathways on ATVs, looking for tracks, Manley said.

The string of package bombs in the city had, before Sunday, left two dead and wounded two others on the city’s east side. Thirty-nine-year-old Anthony Stephan House and 17-year-old Draylen Mason were both killed in the previous explosions. Both victims were connected to two prominent black families in the city, according to the Austin-American Statesman. Their families have “long histories fighting for racial justice and empowerment of the city’s African-American community,” the newspaper reported. The third bomb hit a 75-year-old Latina resident and her mother.

Manley said the latest victims were “two Anglo males,” ages 22 and 23.

“At this point, we have people we have looked at, but there’s no leading suspect at this time,” he said on Good Morning America.

The victims of Sunday’s explosion have not yet been identified, but its location likely means that the attacker—or attackers—have expanded their reach.

Residents throughout the city have been increasingly roiled, and a bomb threat on Saturday led to the cancellation of a Roots concert at the South by Southwest festival. Manley said that more than 500 federal agents are assisting the investigation.

Officials have offered a $115,000 reward for any information leading to the arrest of the bomber.

Sunday evening, Chief Manley made a public plea to the perpetrator to come forward and share their “message.”

“There’s the message behind what’s happening in our community, and we’re not going to understand that until the suspect or suspects reaches out to us to talk to us about what that message was,” Manley said. “We still do not know what ideology may be behind this and what the motive was behind this.”

“These events in Austin have garnered worldwide attention, and we assure you that we are listening,” he continued. “We want to understand what brought you to this point, and we want to listen to you.”