Justice Today

Cold Cases and Serial Killers (Part Two)

December 20, 2023 National Institute of Justice Season 2 Episode 20
Cold Cases and Serial Killers (Part Two)
Justice Today
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Justice Today
Cold Cases and Serial Killers (Part Two)
Dec 20, 2023 Season 2 Episode 20
National Institute of Justice

NIJ Social Science Analyst Eric Martin discusses why the number of serial killers is declining and the factors that deter people from this type of violent offense. He also shares insight into whether society is experiencing an evolution away from serial killing and how NIJ continues to support research in this area.

Mark Greene, the Division Director of the Technology and Standards Division at NIJ, and Lucas Zarwell, the Office Director of NIJ’s Office of Investigative and Forensic Sciences, co-host this conversation about serial killers. Read the transcript.

Listen to Part One.

Reading and Resources from NIJ

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NIJ Social Science Analyst Eric Martin discusses why the number of serial killers is declining and the factors that deter people from this type of violent offense. He also shares insight into whether society is experiencing an evolution away from serial killing and how NIJ continues to support research in this area.

Mark Greene, the Division Director of the Technology and Standards Division at NIJ, and Lucas Zarwell, the Office Director of NIJ’s Office of Investigative and Forensic Sciences, co-host this conversation about serial killers. Read the transcript.

Listen to Part One.

Reading and Resources from NIJ

Other Resources

SPEAKER 1: Welcome to Justice Today, the official podcast of the Department of Justice's Office of Justice Programs, where we shine a light on cutting edge research and practices and offer an in-depth look at what we're doing to meet the biggest public safety challenges of our time. Join us as we explore how funding, science, and technology help us achieve strong communities. 

MARK GREENE: Hello, everybody, and welcome to part two of our NIJ Podcast on serial killers. I'm Mark Greene. I'm the Division Director of the Technology and Standards Division at NIJ. And I'm also joined by Eric Martin, Social Science Analyst in our Criminal Justice Systems Division. Hey, Eric.

ERIC MARTIN: Hello, Mark.

MARK GREENE: And Lucas Zarwell, our Office Director of the Office of Investigative and Forensic Sciences.

LUCAS ZARWELL: Glad to be back, Mark. Thanks.

MARK GREENE: Excellent. I'm glad--I'm glad that we could all get together for part two of this conversation. And in part one, we talked about the definition of serial killers. We talked about sort of the history of serial killers; why it feels like they might be on the rise but they're actually on the decline. Eric, at the beginning of part one, you did say that the number of serial killers are on the decline. Could you, Eric, explain why that is, how we know?

ERIC MARTIN: Sure. Thanks, Mark. Well, I looked into it a little bit and there's been a lot of recent interviews with some noted scholars, investigative journalists that have dedicated a lot of time in this area, namely Enzo Yaksic, Thomas Hargrove, and Michael Amott. They are three of the noted minds as far as I can tell in this area. And they've given some very similar answers if you call all the interviews, and it's all intuitive. I think it's things that are pretty tangible to everybody. And they largely take the form of: better investigative practices since, you know, now compared to the 1970s and '80s is the timeframe we're talking about; advanced forensic methods which Lucas knows a lot about and we kind of alluded to last time; and then also different social practices. We are--we have different social norms that may make us less vulnerable to victimization. So if we want to go through each one in turn, you know, if that's okay with you guys.

MARK GREENE: Yeah, definitely. Yeah, let’s hear about it.

ERIC MARTIN: As far as the investigative practices, we spoke on improvements made about linking cases together from multiple jurisdictions, right? And this most notably came through FBI ViCAP and that's supported by the National Center for the Analysis of Violent Crime, which I, you know, pointed out that NIJ actually had a part in seating back in the early '80s. And, you know, there are jurisdictions that have a much more means at their disposal to be able to communicate with each other, share case information, identify case linkages so that would-be serial killers have less anonymity when it comes to going across jurisdictional lines. There's been--in this vein actually, there's been some research on what they call would-be serial killers. Serial killers who get apprehended early but definitely express motivation that they really desired to kill again. So that would be a group that may have been, you know, part of the active serial killer population that we talked about but those investigative practices apprehended them early.

LUCAS ZARWELL: That's pretty cool because, I mean, really what you're saying is like the fact that the exchange of information has gotten easier is what I hear you're saying.


LUCAS ZARWELL: And that, you know, now we've got--we've got, you know, everything's changed since the '80s, right? I mean, we have cell phones, we have email, we have the internet, all these resources and ways to communicate and ways to socialize that just didn't exist back then. That's a cool, cool point.

ERIC MARTIN: Yeah. And ViCAP was created in part to really help the analysis of violent crime across jurisdictions, so that jurisdictions at different parts of the country can look for case similarities and maybe identify a linkage. There was a perception at the time and it was--there were a few operating in this vein but the long haul truckers or people that were nomadic across the country, you know, would make people vulnerable in different jurisdictions and police really had a hard time catching up.

LUCAS ZARWELL: That totally makes sense. Yeah. Absolutely. 

ERIC MARTIN: And then, going to advanced forensic methods and Lucas, jump in whenever you want, we talked about forensic genealogy.


ERIC MARTIN: Thank you. How Joseph DeAngelo was apprehended, you know, and this start the whole--this was the impetus for, you know, our--all our conversation and our inquiry here.

LUCAS ZARWELL: Yeah, yeah.

ERIC MARTIN: But we've really started to push the envelope of advancing how we can look for matches in CODIS, and there's a lot of stuff going on. NIJ was involved in some research just in what jurisdictions are doing. There is a lot of diversity in this country about what types of searching and analysis is being used. But pretty much I think some of the three types of advanced searching that we have today, there's partial matching, and this is where you could be doing a routine search and it may hit on someone who's a close family member relation to the suspect that left a viable sample. And then there's...

LUCAS ZARWELL: Right, and this is like--that’s in CODIS.


LUCAS ZARWELL: That's actually in CODIS, right? I mean, they have a partial match ability actually in CODIS. So, like, having a family member who is in CODIS, perhaps for a different crime or another--well, it would be for a crime reason if they're in CODIS, right? And so to have that connection is enough to kind of move forward with some idea of who we're looking for. That's really neat, yeah.

ERIC MARTIN: And then a close cousin to that, if you will, is deliberate familial searching. And that's with software packages that really go out and look for family members. And, you know, of course, this is all driven by case information and by the theory of the crime. And then, to wrap it all up again, we have forensic genealogy that I, you know, and correct me if I'm wrong, Lucas, uses a combination of all this, and of course you have the DNA sample that's used for genealogy purposes, people trying to find their ancestry and whatnot and that's boomed since then.


ERIC MARTIN: And I think it’s important to say—oh, go ahead.

LUCAS ZARWELL: No, no. It’s totally fine. I mean, I think right now when we think about it, it’s really an investigative type of technology. Right? So we like to use the term, you know, investigative genetic genealogy because what’s going on there is they’re using a different technology that has, you know, to understand the DNA and to match the DNA with some of these commercial databases which then can reveal members in a family tree. And then, what happens then is after they get some connections, they map that back to a genealogist who actually walks it through like an ancestry.com type of exercise. I’m not a, you know, clearly I’m not a genealogist, but I think the combination of that has been--yeah, it's been shown to be pretty effective in investigating some of these cases out there. I mean, the Golden State Killer obviously for sure, yeah.

ERIC MARTIN: And I think it's important just to note for our listeners, you know, the thousands of you out there right now listening to this podcast, this is all recent innovations, right? We're not talking about the state of DNA searching compared to the 1980s, right? You know, DNA did not--was not even predominantly used for investigative purposes back then at all in the '70s and '80s of course. So we're talking about advancements that have taken place, you know, relatively recently, just so that everybody's aware.

LUCAS ZARWELL: That's right. Yeah. Absolutely. Absolutely. And I think that it's important, you know, we--that we stay out in front of how these technologies are being applied, you know, not only by law enforcement but, you know, across the technological spheres. You know, we're dealing with such a powerful--such a powerful amount of data for identification. You know, the responsibilities of that I think are important to reflect on.

ERIC MARTIN: Uh-hmm. Yeah. And the privacy concerns and yeah. Great point. And then, really quick just going into the last kind of component of what may be attributed to the decline in active serial killers today, different social practices. You know, think of today compared to the 1980s. We don't tend to pick up hitchhikers anymore. We have the rise of what we call the helicopter parent, and I'm definitely included in that. I see, you know, I think of how, you know, my kids are being raised compared to how I was, you know, in the early '80s where I would just be outside largely all day long. And again, this is not to lay blame to any, you know, survivor of a victim or anything. It's just, you know, it's true. We've evolved as a society, and this evolution may be making us less vulnerable to this type of victimization. Also cell phone use. We have cameras and communication that we carry in our pocket. That was unheard of back in the '80s. Yeah, go ahead, Lucas.

LUCAS ZARWELL: Right. And I mean--no, no, I think that's a totally important point. I mean, you've got probably changes--you know, I do remember safety messages as a young man, you know, a thousand years ago, where they were talking about, you know, the dangers of hitchhiking, like why that was kind of--that was mediated out, you know, and accepted and so people deterred from doing it because there was inherent dangers of--that came out of hitchhiking, right? And I mean, I agree. The helicopter parent is kind of a different phenomenon. You know, I myself am--have helicoptered many things and I try to avoid it but I mean, the more information we have, you know, the more we make sure where our kids are. And not to mind you, like right now, like, you know, there's children out there that have cell phones on them and that their parents can know where they are, you know, just from the data that the phones transmit between each other, so you're right. I mean, so technology, social practices, all this kind of goes into that. Great point. 

MARK GREENE: So Eric, I guess--I guess sort of wrapping this up for the--for the listeners out there, I mean, is it--is it primarily would you say deterrence that’s reduced the number of active serial killers?

ERIC MARTIN: Well, this is where things get muddled. And as I was rehearsing for this podcast, and I discussed this with my wife who has a much sharper analytical mind than I do even though I'm the one who's employed as an analyst, she pointed out wouldn't that just reduce the number of victims and not necessarily the number of active serial killers? And I'm like, wow, that's a great point, right? Because if you have these motivations and you have this ideation, you know, do they just have to work extra hard, so to speak? And of course, this would apply later to...

LUCAS ZARWELL: That’s a great point, yeah.

ERIC MARTIN: ...yeah, not necessarily better investigative practices if we're apprehending them early. So I think definitely more research needs to be done in this area, and one thing I really want to focus on a little bit too is when talking about different social practices and how people may be less vulnerable to this type of victimization, that's not clear cut across the board for everyone in our community. There are still certain groups of people that seem to be very vulnerable for predatory practices. Those who have a severe substance use disorder, those who are homeless, and prostituted persons still seem to be very vulnerable to this type of victimization. In fact, Kenna Quinet, she's professor emeritus at IUPUI, she studied serial killers throughout much of her career, and she noted that while the overall victimization rate and the number of active serial killers has decreased the proportion of prostituted persons among victims has increased. Specifically in the 1970s, she identified 16% of known serial killers focused exclusively on prostituted persons. This jumped up to 69% in the early 2000s.


ERIC MARTIN: Yeah. So we really can't, you know, make a blanket claim that we're, you know, less vulnerable because there's still members of our community that are very vulnerable to this type of victimization. And, you know, there's some talk about, well, have--would-be serial killers, have they evolved into mass murderers? Because this is another type of violence, you know, that's kind of new to us today, although it's, you know, it's not new. We've always had mass murderers, but we tend to think of our time period being, you know, seeing this on the rise, although there's a lot of research that needs to be done about trends and whatnot. But this has come up in the literature. Well, are they evolving into a different type of violence, given that it may be more difficult to commit violence as a serial killer? And I think the research on that isn't really showing that with any clarity. It's definitely mixed. 

But there is one thing that stood out to me when I was reading all this. And it came from the Quinet study again. And she quoted a statement given by Gary Ridgway, the Green River Killer. And it really struck me because it wasn't--it kind of illustrated that it's not just because of ease of access as to why prostituted persons were being preyed on in certain instances. Gary Ridgway, at his sentencing, said that he hated prostituted persons and he did not want to pay them for sex. And, you know, there's a number of different motivations, you know, among serial killers and among those who commit mass murder, but I think a common thread might be, in some instances of both, kind of like a violent misogyny if you will. You know, this kind of like anger or hatred, you know, perpetrated against women. I think that might be a common theme. Obviously that doesn't explain everything, right? Right. There's a lot more research that needs to be done but that was one thing that really struck me. And of course, the big takeaway is that there are still many people who are vulnerable within our community.

MARK GREENE: And we'll probably touch on this later in the podcast but I mean, you know, I think you and I have talked in the past about, you know, you find a serial killer, right? It's like that's not--that's not where they started their criminal career. They start, like, way earlier doing much less sort of sensationally violent things and much of that is sexual violence in some cases too.

ERIC MARTIN: There is a whole body of research on trying to identify, like, a criminal career progression if you will. And there isn't one, you know, clockwork stepping, you know, ladder if you will, a career trajectory for these individuals.


ERIC MARTIN: If there was, that would be great. It would be something we could communicate to law enforcement and prosecutors, you know. But there are definite trends, right? And a lot of this research bore out when looking at the case history of Albert DeSalvo, the Boston Strangler. And he started out in burglaries and property crime and then escalated very quickly to violence. And there is, you know, this kind of notion and I was made aware of it when I was a crime analyst with DC Police, so it's been around for a while. But there is this notion of the sexual predator burglar. And there's not a lot of research on it, but I did see a study from Danielle Harris and her colleagues, and she was looking at a sample of people with a sexual offense history and she found out that, in that sample, 33% had a prior burglary charge. So that's a pretty sizable proportion within that sample. Now that's not--it doesn't work the other way. Yeah, it's not like there's a sizable proportion of burglars that have a past sexual offense, but there does seem to be this kind of notion that for some individuals, committing burglary may give, you know, some kind of sensation of breaking, you know, into someone's home, the safety of their domain and that thrill apart from just getting, you know, items to then sell for cash or whatever they want to do.

MARK GREENE: So Eric, I'm going to take a different approach to this. So do you think that there's been some evolution of offending like away from serial killing? Like, is there a trend in a different direction?

ERIC MARTIN: You know, and I think that's what--that's what we don't know at this point. You know, there's certain things that we do know. Those who tend to commit this type of violence have a very diverse offending history and offend early. That is one trend we can kind of pick out. But it's, you know, it's really hard just kind of, you know, even trying to identify the frequency with which some of these people will offend. Like we talked of--at the early onset of the first podcast about, you know, spree killers now being included in the definition of a serial killer. Related to that, NIJ concluded some very fascinating research with Michigan State University. And they--with the help of Michigan State Police that were clearing a backlog of sexual assault kits, so like the forensic evidence collected after reported sexual assault. They were able to use that sample to try to identify those who committed more than one sexual assault within that sample, right? And then, of those who committed more than one sexual assault within the sample, try to look at, you know, is there certain classes of behaviors that you can kind of identify based on frequency of offending? And what was really interesting is that the largest class identified of those who, you know, had more than one offense within their sample committed offenses at a very low frequency over a long period of time. So you could see how that would really inhibit some of law enforcement's ability to detect them. You know, compare this to 6% of the sample committed crimes in quick succession and were apprehended, you know, so I think we're starting, Lucas, to get into be able to identify certain patterns or aspects of offending behavior but I don't think we're at the point yet to identify like, oh, these crimes are becoming more prevalent because there's more attention paid or less vulnerability on this end here. Does that make sense?

LUCAS ZARWELL: It does. It does make sense to me, and I think it's interesting that focus on escalation. You know, that really--that resonates with me when I'm listening to you.


MARK GREENE: So it's interesting though, Eric, I mean, you talk about the sort of the quick escalation and that can lead to a very rapid apprehension but this sort of, for lack of a better phrase, this sort of intermittent defending and this sort of slow burn to a criminal career. It's not, you know, it doesn't really paint a promising picture for sort of being able to apprehend these folks. And it's like, you know, the one guy, the Golden State Killer, he's like, you know, like these people, they're in their old age and they find them, you know, so, you know, what is it that police agencies can do about this if anything?

ERIC MARTIN: That's a great question, Mark. And this was the whole point of our original article that started all this. Pursue cold cases to the extent possible and, you know, I said it in the last podcast, not every agency in this country has a dedicated cold case unit. But research shows that having dedicated investigators to work cold cases and a specific resource stream to support them does help. And also you're--in our studies of cold cases and how they come to closure, investigators who are in the position to--who are looking at these cases, you never know where the leads are going to come from and it seems like from what we saw in our NIJ-funded research that a new investigative lead often produces a windfall effect, right? There may be witnesses that saw something that they did not understand or...

LUCAS ZARWELL: That makes sense.

ERIC MARTIN: ...realize was relevant to the case at the time. But, you know, there may be new information come out, a different theory of the case has developed and all of a sudden, that witness has very pertinent information that, you know, investigators at the time of the crime didn't really concern themselves with--concern themselves with.

LUCAS ZARWELL: Yeah. I mean, because you think about it, I mean, there are people who are more willing to discuss what they might've seen some night because they may not want to have said something out of fear before, right, or there's--now there's new technologies that could be applied to evidence that just hasn't been applied before. You've got this interconnectivity that you discussed earlier and how important that is in terms of just cross-jurisdictional communications. And then, I think just the information, right? I mean, we've got all this information out there at the fingertips of various investigators who are very skilled and there is definitely a skill in being a cold case investigator. And I think all those things, I agree with you, Eric. I think those are--those are good points to highlight.

ERIC MARTIN: And I, you know, Lucas just--you speaking reminded me of another big point by nature of--yeah.


ERIC MARTIN: By nature of your position at NIJ. You--look for any possible retained physical evidence. We talked a lot in our conversation about methods change all the time, you know, the ability to capture a viable sample is much different now than back in, you know, the '80s, '90s and early 2000s. And you might find cases where physical evidence was retained for certain purposes like serology analysis before DNA was in vogue and in use. That--there might still be evidence there, and we have seen that. So, you know, search for any possible retention of physical evidence and see if there's something that could be done with it.

LUCAS ZARWELL: Yeah, totally. I mean, evidence in itself is very, very valuable and the methods we have today are much more sensitive and strong than they were in the '70s, so...

MARK GREENE: I mean, that's a--that's a really--that's a really interesting point. You know, there could be evidence like in a bag in a locker somewhere at a police department they never swabbed it for DNA back in the '80s or '90s because they just didn't have the capability, and it's like, “oh, wait, hang on. We never did any DNA on this one.” It's like voila. No, it's a really great point. That's a great point.

ERIC MARTIN: And I think, you know, just to conclude and this, you know, and moving away from just law enforcement, you know, to just speaking to everybody in the community. You know, the one thing we're sure of the number of active serial killers is down today from what it was, you know, 30, 40 years ago. But there are still people in our community that may be vulnerable to victimization. So I think, you know, just to end our conversation, I think it's important to--for all of us to still be vigilant, you know, cooperate with law enforcement when they need our help and just look out for each other, you know, look out for everybody in the community.

LUCAS ZARWELL: Yeah, completely, especially the most vulnerable, right? I think that's very important because that seems to be--that seems to be what--where the evidence lies right now, and I think that's important to highlight.

ERIC MARTIN: Yeah. Good point.

MARK GREENE: Right. Easy, you know, people who are, you know, like you said, substance use disorders, homeless people on the streets, easy targets for somebody. That's absolutely right. Yup.


MARK GREENE: Yup. Great. You know, guys, I've really appreciated having this conversation with you and being able to actually extend it into two podcasts. I think this has been really great. I hope the audience has enjoyed hearing about serial killers on this NIJ podcast. Thanks to Eric and Lucas for joining me to talk through this very interesting topic, and I hope everyone will continue to listen to NIJ podcasts in the future.

SPEAKER 2: To learn more about today's topic or about NIJ, visit the links in the episode description and join us for new episodes every month. 

Opinions or points of view expressed in this episode represent a consensus of the authors and do not necessarily represent the official position or policies of the U.S. Department of Justice. Any products and manufacturers discussed in this episode are presented for informational purposes only and do not constitute product approval or endorsement by the U.S. Department of Justice. 



Introduction: Why are serial killers on the decline?
Better investigative practices
Advancements in DNA technology
Social practices have evolved to hinder serial killers
Can we predict if an individual will commit serial homicide?
Are other crimes on the rise, while serial killing is declining?