AUSTIN, Texas (CBSNewYork/AP) — Police in Austin, Texas say preliminary information shows similarities between an explosion that injured two people Sunday night and three previous explosions earlier this month elsewhere in the city.

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.

“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,” Police Chief Brian Manley said Monday. “Again, this is preliminary information, but we have seen similarities.”

Manley said “the big difference” in the latest device is that police believe a tripwire was used.

“The belief that we are now dealing with someone who is using tripwires shows a higher level of sophistication, a high level of skill,” Manley said.

The latest blast occurred around 8:30 p.m. Sunday in a suburban neighborhood known as Travis Country in southwest Austin — far from the previous three that were all in residential areas in the eastern part of the city.

Two men in their 20s were hurt in the latest blast. Police said they were hospitalized with injuries. It was the fourth explosion to rock Austin in less than three weeks.

The first was a package bomb that exploded at a northeast Austin home on March 2, killing 39-year-old Anthony Stephen House when he opened the package left on his doorstep.

Another package bomb then exploded farther south on March 12, killing 17-year-old Draylen Mason and wounding his 41-year-old mother when she opened a package that arrived at their home.

A 75-year-old woman was also hurt in a package explosion that same day.

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 were black or Hispanic, but now says that investigators aren’t ruling out any possible motive.

“So now, what we’re imploring the community to do, if you see any suspicious object or item that looks out of place, do not even approach it, but instead call 911,” Manley said.

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.

Police kept residential streets near Sunday night’s blast on lockdown, gradually expanding their barricades.

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 late morning.

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.”

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 also be reassured by the massive police response to the attacks. Droves of federal agents are investigating, along with Austin police.

“That anxiousness is going to continue until we can find the answer,” Adler said.

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 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.

“It’s a real situation now,” said resident Crystal Offutt. “It’s not, ‘Oh don’t touch any packages.’ It’s like, ‘No, seriously!’ If you see anything weird, stressing it your children this is something that happens.”

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.)

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