AUSTIN -- Austin police are now saying the trigger in a fourth explosion in Austin in the past month wasn’t a package, but instead a tripwire.

"All the tripwire is, is a means to initiate the device,” said Kevin Walter, a previous Explosive Ordinance Disposal Technician for the U.S. Army.

And even though that initiator is different this time, the experts like Walter say it’s the same idea: a victim initiated a trigger.

"One thing common so far, is these are victim operated devices, so these are devices meant to function like booby traps -- you put a device in place, and you set it up in such a way that you know a victim is going to initiate something when they pick it up, or, like you mentioned. when they trip over it,” said Aaron Forum, a former Special Forces and Green Beret service member.

The experts said these bombs aren't necessarily difficult to make.

“Absolutely no education on this stuff is needed. You can find it either on YouTube, online, in certain places, it’s nothing you need specially training for,” said Kem Lance, who was trained in explosives by the U.S. Army. “When it comes to explosives, all you need is a good imagination and explosive material and an initiation device.”

Even the tripwire, could be made out of simple materials like fishing wire and popsicle sticks.

"Bombs as destructive devices themselves are actually pretty simple -- they use simple materials, they use simple chemicals or compounds,” said Bernand Zapor, a retired Supervisory Agent in Charge at the ATF.

They also said they're not necessarily difficult to transport to the various locations.

"Some sort of mechanical device that allows you to maneuver the package anyway you want and it's not going to, it won't set it off, because it has an open circuit. So once you place the package, there's some way you can close the circuit real quickly, walk away, and they're totally safe from the package, and the next person who comes in and disrupts it, or triggers it in any way, is going to be injured or killed,” said Walter.

But they each said there is something else that alludes to the idea that this person does have some expertise.

"I think the fact that we've seen four devices in place, and all initiated reliably and the in-placer didn't have any issues at all in placing them and getting away with it, and there are no accidental initiations, I think that would suggest a higher level of training,” said Forum.

"Oftentimes, criminal bombing cases in the United States, are actually an investigation of someone who was attempting to make a destructive device and it deployed, detonated in the process of manufacturer,” said Zapor.

And with each so-called successful explosion, Zapor said the bomber could keep doing it.

"If someone is going after multiple targets, if these events are in fact related, they tend to perpetuate, and people become more sociopathic with time, and they're gaining greater joy you could say with this type of event and the notoriety that comes from it -- it can become addictive, compulsive behavior,” said Zapor.

Reports have indicated that the suspect or suspects have added nails, bolts or pieces of glass to the bombs.

"The addition of nails or bolts or any of these kinds of things definitely shows that it was intended to kill,” said Zapor.

The experts said your eyes are your most powerful tool. They urge everyone to be aware of their surroundings and report anything suspicious.