(CNN)The sophisticated technology and method used in the fourth explosive device in Austin, Texas, this month suggest that whoever made the bomb knew what they were doing, authorities and experts say.
What the tripwire explosive in Austin suggests about the bombmaker
But they say the explosives reveal almost nothing about the bomber's ideology.
Three parcel bombs exploded at homes in the Texas capital over 10 days, killing two people, wounding two others and leaving a community shaken. On Sunday night, a device that police believe used a tripwire exploded in a residential area and injured two men, police said.
The suspected tripwire explosive had similarities to the previous three explosives, but it was more sophisticated, authorities said.
As state and federal agencies work together to find answers, here's what experts say the explosions tell us about the culprit or culprits.
Making a bomb that works at the right time is harder than it might sound.
The three parcel explosives in Austin detonated while a victim was carrying them, and not while the suspect or suspects was placing them on the victims' doorsteps.
"That shows that the person who's doing this, they know what they are doing and they've probably practiced a lot," Ben West, a security analyst with the geopolitical intelligence platform Stratfor, told CNN affiliate KXAN.
It often takes several attempts for parcel bombers to "hit their stride," and they "are rarely this effective" on the first try, a report released by Stratfor said.
"We are clearly dealing with what we expect to be a serial bomber at this point," Austin Police Chief Brian Manley said on Monday.
The explosive device which may have used a tripwire shows the bombmaker's expertise and training, according to Stratfor.
"The device's success, despite significantly different design, further suggests that the bombmaker behind these attacks is an accomplished one, and has likely to have received some training, perhaps as a military or police explosive ordnance disposal technician," Stratfor said.
The suspect either lives in the Austin area or is working with someone who lives in the Austin area.
The explosive packages were not delivered by the US Postal Service or any other mail delivery service, so someone hand-delivered them, likely overnight, to the three Austin homes, the Stratfor report says.
Authorities are likely looking at surveillance video from neighbors' yards that may have captured any vehicles or people going to and from the area, said Ryan J. Morris, founder of Tripwire Operations Group, a company that provides explosives training to law enforcement.
Sunday's explosive was concealed on the side of the road in a residential area, and it took place in a different part of the Austin than the previous three blasts.
Not many details about the construction of the bombs have been made public, but Chief Manley said "these are very powerful devices."
"So there's a certain level of skill and sophistication that whoever is doing this has, and ... we are hoping to use the evidence we have to track them down based on what we are seeing on all three scenes that seem to be consistent," Manley told KXAN on Tuesday.
The suspected tripwire explosive suggests that the bombmaker or makers have a "higher level of sophistication than maybe we initially thought," Manley said.
The way the bombs were detonated could indicate how skilled the maker is.
"The detonation of the device when it was moved could indicate the use of a remote detonator," the Stratfor report said, or the use of a triggering device such as a mercury switch -- which is activated by movement.
If the bomb maker used a remote, it would have "required the bomber to have had visual contact with his victim -- potentially exposing him to detection," the Stratfor report said.
After Sunday's explosion, a Stratfor report said that the bombmaker's versatility means future bombings could use other methods, such as infrared beams, pressure plates or remote control.
The victims in the three parcel bombings were Hispanic or African-American, a fact that left people of color in the community feeling threatened. But the victims on Sunday's explosion were two white men, leaving open the question of whether the victims' identities matter to the bombmaker.
The tripwire device used in Sunday's explosion targeted random victims who just happened to be walking past, police said.
Morris, of Tripwire Operations Group, said tripwire type devices are not targeted to any specific type of person.
"You could be of any kind of background. That's nondiscriminatory," he said. "This was just meant to target anybody, obviously just to make a point."
But what is that point? Police have not discovered a motive in the bombings and have not been willing to classify the explosions as hate-related.
"We said from the beginning, we're not willing to rule anything out. Because when you do, you limit your focus," Manley said. "This changes the concerns that we had initially. Though we have still not yet ruled it out."