Authorities are continuing to search for suspects and motives on Monday after the city of Austin, Texas was rocked by a bombing on Sunday nigh—the fourth this month.

The latest explosion in the Texas capital took place just after 8:30pm local time Sunday in the neighborhood of Travis County, a residential area in the southwest part of the city. Austin Police Chief Brian Manley told ABC‘s “Good Morning America” Monday that, unlike the previous three explosions which took place in east Austin, this latest attack was triggered by tripwire, and showed “a different level of skill.”

Another difference from the previous explosions, Manley said, is that the victims from Sunday night’s attack were white.

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Speaking to press late Monday morning, Manley did not say the attacks were being classified as domestic terrorism, but it is clear from law enforcement officials and experts that they fear this is the work of a “serial bomber.”

The latest victims were two men in their 20s, who were riding or walking bikes and were injured when a package left on the side of the road detonated. Their injuries were not life-threatening, but were reported as serious.

The Austin American-Statesman reports:

Still, the police believe that the four explosions are connected, and Manley said Monday that the suspect or suspects “had a higher level of sophistication” than authorities first thought.

Speculating that it’s the same person or persons behind the four explosions, Fred Burton, a security and terrorism analyst at Austin-based Stratfor, told the Statesman that “this is a person very familiar with bombs.”

Earlier on Sunday, the Austin police announced that the reward for information leading to the arrest and conviction of person(s) responsible for had increased to $115,000. Manley said at an afternoon press conference, “We hope this person or persons is watching and will reach out to us before anyone else is injured or anyone else is killed.”