Police in Austin, Texas, reported that the city’s fourth bomb in less than a month shows similarities to the previous blasts but indicates a “significant change” in the “serial bomber’s” tactics and level of sophistication. Sunday night’s device appears to have been detonated by a tripwire when two young men walked nearby.

Austin Interim Police Chief Brian Manley told reporters Monday morning that investigators believe the use of a tripwire in the fourth bomb shows a higher level of sophistication than already determined. He said it also indicates a shift in the “serial bomber’s” strategy of targeting specific people to a more random style.

Briefing regarding explosion in Southwest Austin https://t.co/j2HwQsrse8 — Austin Police Dept (@Austin_Police) March 19, 2018

“Based on the preliminary investigation we have done at this time, we have seen similarities in the device that exploded here last night and the other three devices,” Manley explained. “The big difference in this device is we believe that a tripwire was used in this device.”

“We are clearly dealing with what we expect to be a serial bomber at this point, based on the similarities between now, what is the fourth device.” The chief said they have not yet been able to determine if there is any particular purpose behind the attacks.

The chief said they have cleared the area where Sunday night’s bomb exploded.

Manley asked residents of the Travis Country subdivision who might have video surveillance cameras installed on their homes to contact police so that they can evaluate the video for possible clues about the bomber or bombers.

ATF investigators explained the increased sophistication of the fourth bomb.

“This device is a little more sophisticated than what we have seen to date,” ATF Special Agent in Charge Frederick J. Milanowski told reporters. “A tripwire is a victim-actuated switch. It literally utilizes some type of wire and when you step your foot on that wire, it activates a deadly device.”

He said it can be activated by either “tripping over it, or picking up the package–any tension that is put on that wire. If you move that package or step on the wire, it is likely to detonate.”

Officials emphasized that people seeing a suspicious package must not touch or approach the package. Instead, they ask that people call 911 and ask for help. Bomb dogs and technicians will be dispatched to the area, officials stated.

FBI Special Agent in Charge Christopher Combs said they are very concerned about the change in strategy.

“We are very concerned that with tripwires, a child walking down the sidewalk could hit something,” Combs said. He explained that the use of a tripwire shows the bomber is not targeting an individual but is seeking random victims.

Chief Manley stopped short of calling the attacks acts of domestic terrorism.

He said Sunday night’s bomb was located near a fence and was detonated when the two victims walked by. He described the victims as white males, aged 22 and 23. Manley said the victims were seriously wounded but are expected to recover.

“As we said at the beginning, we are not willing to classify this as terrorism, or as hate,” the chief explained. “We just don’t know enough. And what we have seen now is a significant change from what appeared to be three very targeted attacks to what was last night, an attack that would have hit a random victim that would have happened to walk by.”

All three law enforcement officials reminded the public about the reward being offered for information leading to the arrest of the bomber or bombers. That reward climbed to $115,000 on Sunday, hours before the fourth bombing attack, Breitbart Texas reported.