The Austin Police Department is the public face of the investigation into the four bombings in the city this month. But behind the scenes the APD is getting help from the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) and the FBI.

Alain Stephens, an investigative reporter with the Texas Standard, has been speaking on background with some ATF officials. KUT's Nathan Bernier talked with Stephens about how the agency investigates bombings.

KUT's Nathan Bernier talks with Texas Standard's Alain Stephens

The following transcript has been lightly edited for clarity.

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Nathan Bernier: How does the ATF go about investigating this kind of thing?

Alain Stephens: The first thing that the ATF is going to do with any explosion is they're going to be concerned about secondary devices.

This is something that came up in the Department of Homeland Security back during the '90s where they found an emerging trend. What would happen in these explosions is first responders would show up and then somewhere on scene a secondary device would go off and hurt the first responders. They would find that first explosion would actually be a lure for those first responders to show up.

So, right now when they're having these explosions the first thing on the minds of law enforcement officials is to secure that scene and be on the lookout for these secondary devices. That's why it seems like they may be moving somewhat slowly and they're doing these lockdowns is because they're concerned about another explosive device on scene.

Bernier: Once they secure the scene and start looking at the blast, what kind of evidence are they gathering and what are they looking for?

Stephens: The first thing that many of these ATF sources ... [say] is in these investigations they're looking for what they call the "signature." The signature is the packaging, the trade craft, used to make these explosive devices. Most bombers go with what they know. You don't see a lot of different changes in the types of explosives. Typically, they'll use the same few types of explosive devices that they are comfortable with, that they've had a high success rate with. And that's what they kind of stick to.

The success rate is the other thing that they're looking for. If a bomber has the ability to get an explosive device to their target without having it detonate on them, if they're able to get the bomb to do what they want it to do, they call that the "success rate." And they look at that as well to see, "OK, this person seems like they know what they're doing."

And again, those are all kind of factors that help with their investigation.

Bernier: Right, because it seems like a pretty sophisticated device. Would it require a lot of skill to manufacture these kinds of bombs?

Stephens: A lot of people are very quick to point that this person must have some sort of military background, but ATF has told me this is not necessarily the case. They say that there's a lot of information online that can really help someone create one of these devices if they really wanted to do so.

Bernier: Is there a database they could use to try and link different bombings if they've happened in different parts of the country?

Stephens: So there actually is. Since 1976, the Bureau of Alcohol, Tobacco, Firearms and Explosives have been tracking these explosive incidents. But in 2004, the Attorney General's office would create the Bomb and Arson Tracking System known as BATS. And this has about 400,000 documents, leads, patterns, trends of explosive devices that law enforcement from across the country interact with.

What they do with that is they look for trends. They say, "OK, if we saw a packaged bomb with a tripwire like this, does this kind of match something that maybe we've seen 10 years ago in another state?" And they use that to create leads and patterns.

This somewhat emerged from a bombing that occurred in 1989. A bomber killed a federal judge, Robert Smith Vance. What happened was they would find an intact explosive device. They diffused that device and a ATF analyst actually found out this device was similar to something they diffused 17 years prior. It was that information that would lead to the arrest of Walter Moody who was the bomber. So this is kind of how they operate when it comes to tracking down these leads with explosives.

Bernier: And how many investigations are they doing each year on explosive devices?

Stephens: There's an ATF report from 2016 – that's the most recent one tracking explosive incidents – and the 2016 report shows that there's about 699 explosive incidents. Now some of those do account for accidental explosions. They said that out of those 699 about 439 were bombings. That was an 11 percent increase from the year prior, which was 2015. But still those numbers were quite low and had been trending lower. In 2012, for instance, there's about 1,242 explosive incidents. We saw those numbers dip. There's also some other interesting information that they found out. One of those is that the main charge for a lot of these explosives is actually a pyrotechnic mixture, or fireworks.

Bernier: Really?

Stephens: Yes.

Bernier: So the most commonly used ingredients to cause the explosion is fireworks?

Stephens: Yes, that's what they were saying. They said that was overall, the most-used main charge for a lot of explosives.

Bernier: Are there any other investigative methods that the ATF might be using to try and locate this person or people?

Stephens: Yeah, as we mentioned before we were talking about success rates. When we see the explosion that hits a victim, this is typically not the first time this bomber has assembled an explosive device. One of the things that law enforcement is going to be doing is looking for possible test explosions in the area where a bomber may have created a device in preparation and gone out into the middle of nowhere and tested it. So they're going to be combing through all these calls and reports of explosions possibly in the surrounding area.

Bernier: Alain Stephens is an investigative reporter for the Texas Standard providing us some context on how the ATF goes about investigating these types of explosions.

Alain, thanks for your time.

Stephens: Thanks for having me.