Last weekend, a fourth bomb went off in Austin, Texas, the Washington Post reports.

The first three bombings in Austin shook the city and its residents. All three of those bombs were delivered in packages, and the victims of the first bombings were people of color. Despite the race the victims, police said they weren't deeming the attacks a hate crime. Following the first three explosions, police received over 150 calls regarding suspicious packages. 

The latest bombing happened in a different fashion. On Sunday night, a trip wire triggered a bomb in southwest Austin, injuring two white men, according to KXAN. A St. David's South Austin Medical Center spokesperson confirmed that the two men are in good condition but said that both sustained significant injuries. 

"We are clearly dealing with what we expect to be a serial bomber at this point," said Austin Interim Police Chief Brian Manley, who believes that the newest bombing is connected to the first three. Manley said that although the first three bombs showed a high degree of expertise, the newest bomb shows "a higher level of sophistication." 

"It was loud enough to hear inside of our house with our windows and door shut. Again, airplanes go by and cars backfire so we didn’t think much of it," said Stan Malachowski, who lives in a home nearby.

Manley noted that the bomb was left near the side of a road which is different from the front porch location of the other explosions. It also believed that the bomb may not have been inside of a package. 

The newest bomb was placed not in a minority area as the first ones were but in a wealthy, predominately white neighborhood. 

Councilwoman Ellen Troxclair, who represents the neighborhood, said residents are scared.

“It’s shocking,” Troxclair said. “The trip wire definitely instilled some fear into this neighborhood. They just want to know what’s going on.”

"We now need the community to have an extra level of vigilance and pay attention to any suspicious device, whether it be a package, a bag, a backpack, anything that looks out of place," Manley warned. "And do not approach it. Again do not approach items like that, but instead call 911 to report it so we can go out and make sure it’s safe."

Chief Manley also called for the perpetrator or perpetrators to come forward.

"We want to understand what brought you to this point and we want to listen to you," said Manley. The reward amount for any information or tips leading to the arrest of the suspect(s) has been increased to $115,000.