AUSTIN (CBSDFW.COM/AP) — Over the weekend Austin was rocked by the fourth bombing this month. Investigators believe the blast, that injured two people walking along the side of the road, was triggered by a tripwire.

During a Monday morning press conference Bureau of Alcohol, Tobacco and Firearms (ATF) Special Agent Frederick Milanowski explained, “Tripwire is a victim-actuated switch that literally uses some kind of wire and when there’s pressure put on that wire it activates or detonates the device.”

Police Chief Brian Manley said the men who were injured in Sunday night’s explosion in the southwestern Austin neighborhood of Travis Country — a 22 and 23 year old white male — are still hospitalized. The victims in the three earlier attacks were black or Hispanic. CBS News correspondent Omar Villafranca spoke with a first-responder who said one of the victims had a nail in his wound, but neither of the victims injuries are life-threatening.

The first three attacks involved package bombs left on people’s doorsteps, but with this explosion being triggered by a wire questions about domestic terrorism were posed to the Chief. “We’ve only gotten into the preliminary phases [of the investigation] and as the day moves on that is something that we’re going to analyze,” he said. “We are clearly dealing with what we expect to be a serial bomber at this point — based on the similarities between what is now the fourth device.”

Authorities on Monday 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 before 10 a.m. Police said the Travis Country neighborhood is still locked down, for safety reasons, until at least 2 p.m. today. Bomb technicians from San Antonio and Houston are en route to help with the investigation.

Chief Manley said, “There’s still a significant amount of evidence, as you can imagine with a blast scene like this the evidence is strewn across quite a significant distance and it’s gonna take us a while to methodically go through and collect this evidence.”

Travis Country is far from the sites of the earlier bombings, which occurred over two-plus weeks in residential neighborhoods east of Interstate 35, which divides the city.

At a news conference hours after Sunday’s blast, which happened around 8:30 p.m., Manley repeated his public warning for people to not pick up or approach suspicious packages.

“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.”

Before daybreak Monday, Austin police pushed another alert to cellphones advising residents to continue staying indoors and to call 911 if they needed to leave their homes before 10 a.m. Austin’s school district announced that buses wouldn’t be going into the Travis Country neighborhood because of police activity and that any “tardies or absences due to this situation will be excused.”

The first of the four explosions to hit Austin this month was caused by a package bomb that detonated 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.

Today Manley again urged residents receiving unexpected packages to call authorities without touching or opening them and said outright that the probability is high that the bombings are linked. “We have seen similarities in the device that exploded here last night and the other three devices that have exploded in Austin starting on March 2,” he said. “Again, this is preliminary information but we have seen similarities.

The latest explosion happened just hours after Austin police made a public plea for help in the case. At a Sunday afternoon press conference Chief Manley made a public appeal for the bomber/bombers responsible to contact authorities. During the same update it was also announced that authorities had increased the reward amount by $50,000, bringing the total to $115,000 for information leading to the arrest of whoever is responsible for the first three explosions.

At the Monday morning press conference officials again asked for help from the public. “If you have video surveillance on your house, whether it be surveillance cameras, Nest cameras… anything like that, we want to get your video footage so that we can have that analyzed and identify any potential persons, vehicles or anything that may be of interest to this investigation,” they said.

Sunday also was the final day of the South By Southwest music festival, which draws hundreds of thousands to Austin every March. It is 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.

(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)