Texas’ capital city was rocked by the fourth bombing this month, which injured two people and which the police chief says was caused by a tripwire and showed “a different level of skill” than the package bombs used in the three prior attacks.

Police Chief Brian Manley also told ABC’s “Good Morning America” on Monday that both men who were injured in Sunday night’s explosion in the southwestern Austin neighborhood of Travis Country are white, unlike the victims in the three earlier attacks, who were black or Hispanic. He said the men were riding or pushing bicycles when the explosives detonated, which differs from the first three attacks, which involved package bombs left on people’s doorsteps.

Authorities on Monday were canvassing 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. After sunrise, officers were canvasing the area for anything suspicious.

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.”

Manley also said authorities had worked to “clear” a suspicious backpack found in the area that was part of a separate report.

Police kept residential streets on lockdown, gradually expanding their barricades and closing off all roads into the neighborhood. 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 two men injured on Sunday are in their 20s and are white. Police said they were hospitalized with injuries that weren’t life-threatening.

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.

Police said all three of those were likely related and involved packages that had not been mailed or delivered by private carrier but left overnight on doorsteps. Manley originally suggested they could have been hate crimes since all the victims of the first three explosions were black or Hispanic, but now says that investigators aren’t ruling out any possible motive.

Manley last week urged residents receiving unexpected packages to call authorities without touching or opening them, and police responded to hundreds of calls about suspicious packages but didn’t find anything dangerous. He continued to make similar please each time he spoke with reporters Sunday and Monday.

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.

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.

(AP)