Interview with an Expert

The increasingly diverse and challenging
discipline of trace-evidence analysis.

An exclusive Evidence Technology Magazine interview with
Robert D. Blackledge
Forensic Chemist (retired) with the Naval Criminal Investigative Service (NCIS)

EVIDENCE MAGAZINE: Trace evidence is a very broad field, isn’t it? Hair, fiber, glitter, dirt, paint…

BLACKLEDGE: Yes, it is actually a very diverse field. Almost anything can be trace evidence. Some things are not called trace evidence by most people, although I could probably make an argument that they are trace evidence. For example: If you walk across a room, you are going to either leave traces of your footprints because of dust on your shoes or you are going to be picking up traces of the dust on the floor. Most people would just call that footwear impressions. But really, it is an exchange of contact trace. That’s why there needs to be more emphasis on training the people who process crime scenes so they will be more aware of the possibilities—so they will be thinking of trace evidence right from the start.

EVIDENCE MAGAZINE: You have been doing this for quite awhile, right?

BLACKLEDGE: I got started in this area back in 1971. And Yes, it has changed significantly over the years! Back then, if I had an area of evidence that I did not know much about, there might have been some people I could have called. But I couldn’t just sit down at a computer and Google the topic like we all can do today to find the key references and resources that we need.

EVIDENCE MAGAZINE: How did you get started in your study of the trace- evidence discipline?

BLACKLEDGE: Actually, I spent most of my time doing drug analysis. I was with the Naval Criminal Investigative Service (NCIS) forensic laboratory in San Diego, California. But most of our military cases were adjudicated before going to court martial and requiring testimony. That’s why I had time to get involved in trace evidence and do a little bit of research.

EVIDENCE MAGAZINE: Trace evidence was your specialty, wasn’t it?

BLACKLEDGE: Well, trace evidence has always been my major interest in forensic science. But it was more of a sideline. In most small labs, you can’t really have a full-time trace-evidence person. So you usually end up with one person doing trace evidence part-time.

EVIDENCE MAGAZINE: From your point of view, what is one thing people need to know about trace evidence?

BLACKLEDGE: Consider this: With trace evidence, you have a questioned sample and a known sample. And you are trying to prove that the questioned sample could not have originated from the same source as the known sample. So you focus on that—because as soon as you have your proof, that particular examination is over. If you have a series of tests that you can run, you probably will want to do the easiest ones first, along with the non-destructive ones. As soon as you have a test where known and questioned are different, you have an answer to your basic question: No, it could not have come from a common source. You have eliminated at least that evidence as having any probative value with regard to that particular suspect. Of course, it might have probative value for some suspect who has not been introduced yet.

EVIDENCE MAGAZINE: Are you trying to clear the suspect…?

BLACKLEDGE: People talk about the need for forensic scientists to be unbiased. Well, in a way we are sort of working for the suspect—or at least, for the person who has come up as a person of interest. Why? Because if we can show that the questioned sample and the known sample could not have originated from a common source, then it does not show an association between the suspect and the victim or the suspect and the crime scene. That’s the way every trace-evidence examiner works. You are trying to find a difference. And if, after you have run every test that is practical, you still have not come up with any differences that can’t be explained, you say that the questioned sample could have originated from the same source as the known sample. You can’t say that it did…unless you have come up with something like a fracture match or something else that proves it had to have come from that source.

EVIDENCE MAGAZINE: So you always have to keep an open mind?

BLACKLEDGE: Absolutely. We have to keep an open mind—even if it proves the person of interest is lying. Then we have to ask, “Are you lying because you committed the crime…or are you lying because you are having an affair and you are meeting your lover at some clandestine location and it’s going to ruin your marriage if your spouse finds you do something to hide the facts?” That is something that has to be considered: The person could be lying…but for reasons other than the crime you are investigating.

EVIDENCE MAGAZINE: So are you saying that the real probative value of trace evidence is revealing the possibility of a connection?

BLACKLEDGE: Generally speaking, it is not a single trace that is so important. It is that you have several different traces that you cannot eliminate as having come from the known source. And the more you have, the better the probability that they did, in fact, come from the known source. Someone might say, “Well, this is just circumstantial evidence.” Well, if you have enough of it, the odds of it coming from any other source are pretty small.

EVIDENCE MAGAZINE: Don’t defense attorneys like to pick on you guys who work the trace evidence?

BLACKLEDGE: Yes. They do. And that points out an area that needs a lot of work in forensic science. In many cases, the statistics are not available. For example: I have done quite a bit of work on glitter evidence. The average person on the street thinks that glitter is glitter and it is all pretty much alike. But in actuality, there is a tremendous variation between glitter products. One company in New Jersey, for instance, states on its website that it has more than 20,000 varieties of glitter. With most transfers—whether it is fiber or glitter or paint or what have you—it is difficult to come up with an estimate of how rare a particular sample might be. If I just go out in the community and sit on seats in public places or ride in public vehicles, what are the odds that I would have an exchange and this type of trace evidence would be on my clothing? Right now, it is hard to come up with even ballpark numbers. With DNA evidence, of course, you have very reliable and impressive statistical numbers. We need more research and more statistical information for items that appear as trace evidence.

EVIDENCE MAGAZINE: Isn’t there any way to narrow it down?

BLACKLEDGE: You could probably go by reasoning. If you look at the more common fibers, you might have an idea of the chances of picking up those fibers in a particular color. But for a new fiber, the odds would probably be quite a bit less. Of course, you have little pockets in the logic. For some reason, a certain fiber might be quite common in one location—but in the whole region, it might be quite rare. Investigators and forensic scientists need to know the context.

EVIDENCE MAGAZINE: What do you mean by “context”?

BLACKLEDGE: Let’s say you come up with a match and you’re ready to testify in court. But if for some reason, your agency didn’t want you to be biased, so you were just asked, “This questioned fiber and the known fiber: Do they compare? Are they alike or different?” If you have no idea of the context in which you are doing this, you might say based on all the tests you ran, “Yes, they match.” But if you knew the context, you might realize that this match is not all that unusual considering the area where the fibers were found.

EVIDENCE MAGAZINE: Please explain.

BLACKLEDGE: Let’s say there is a coal-fired power plant. The soil downwind from this power plant has collected some combustion by-products that were emitted from the stack of the plant. In this scenario, the crime scene was also located downwind of the plant. Well, if you are just looking at two soil samples
—one from the suspect and one from the crime scene—and you find that the traces on his shoes and in his car match those at the crime scene, then you might say, “This is really important.” But what if the suspect works and lives downwind from the power plant? In this context, it would not be unusual for the two samples to be alike. In other words, if you had known the context, you would have realized that the match really doesn’t prove anything.

EVIDENCE MAGAZINE: But isn’t trace evidence a key part of investigation?

BLACKLEDGE: Trace evidence can be very useful, of course. Sometimes, a person will give an alibi and the trace evidence will prove that the alibi just doesn’t pan out. It will prove that the person could not have been where he claimed to have been at the time of the crime. Actually, trace evidence can work both ways: It can help to prove that you are innocent…and it can help to prove that you are lying.

EVIDENCE MAGAZINE: What is in the future for trace evidence?

BLACKLEDGE: I think the role of trace evidence cannot help but grow. Other investigative disciplines are becoming more automated and more routine—and rather uninteresting. For someone who has advanced degrees and likes variety, trace evidence could really be the direction to go. Whether you are an investigator working for a police agency or a scientist working in a crime lab, trace evidence offers a wide range of intellectual challenges.

EVIDENCE MAGAZINE: Is there anything new on the horizon?

BLACKLEDGE: Well, there is one thing that I think will become more and more important in the future: stable isotope ratio mass spectrometry or IRMS. For example: Remember the anthrax scare? The white powder? By looking at the stable isotope ratios, they were able to determine that this material did not have a Middle East origin. Instead, it most likely came from the northeastern part of the United States. In doping cases with athletes, they can use the IRMS to look at the carbon-12 to carbon-13 ratio and they can tell whether testosterone from the urine sample was produced in the athlete’s body or whether it came from a source outside the body. I consider that to be an example of trace evidence, even though someone else might say, “No, that is toxicology.” In the future, I expect stable isotope ratio mass spectrometry to become very important in trace evidence.

EVIDENCE MAGAZINE: Does IRMS require expensive equipment?

BLACKLEDGE: Well, it requires a very specialized mass spectrometer where you are only looking at several things: carbon-13, carbon-12, oxygen-16, oxygen-18, hydrogen with a mass of one, hydrogen with a mass of two, chlorine, sulfur—and that’s about it. It is very specialized, but this sort of thing is being done routinely by researchers at the university level. I think it is just a matter of time until the FBI will be doing it routinely, along with some of the larger crime labs. It will never be done by smaller crime labs, but it will be something that can be done as a service for the smaller crime labs.

EVIDENCE MAGAZINE: What would you tell someone who is thinking about a career in forensic science?

BLACKLEDGE: If they were thinking about forensic science as a career or in the early phases of their career, I would ask them to think about what kind of a future they want. So many things can be automated today. Take DNA, for example. Other than the collection of the DNA evidence at the crime scene and the initial preparation of samples for analysis, the DNA analysis part is becoming more and more automated. And that automation probably requires a technician, rather than someone who has a PhD in molecular biology. In the future, most labs will have someone with that degree and they will be in charge of the section. But there won’t be very many others with that type of education. Everyone else will be a technician—and what they do will be very repetitive. To me, it would be like going to work on a factory assembly line. It won’t be very exciting. But trace evidence—by its very nature—defies automation. Why? Because each case is different. Traces are different. Collection methods are different. The way you characterize trace evidence and compare your questioned sample with your known sample... Well, I just don’t see it ever being automated. Some things can be automated, but not trace evidence. To me, that is what is exciting. Every case is a mystery just waiting to be solved.

EVIDENCE MAGAZINE: Is it true that not too many agencies have people who specialize in trace evidence?

BLACKLEDGE: I can’t give you any statistics, but I have read papers that said some law-enforcement agencies are eliminating their trace-evidence sections. On the other hand, I think the pendulum is starting to swing the other way. Why? Because two years ago, the NIJ and FBI co-sponsored a trace-evidence symposium that was very well attended. People from all over the world attended and gave presentations. Now, just two years later, those same two agencies are co-sponsoring another trace-evidence symposium. It will be in Clearwater Beach, Florida from August 2 through 7, 2009*. I was very pleased to learn of this national interest in trace evidence. For some time, DNA and fingerprints were getting most of the attention and trace evidence was being ignored. This new widespread interest in trace evidence is very encouraging.

EVIDENCE MAGAZINE: Thank you for speaking with us today.

A brief look at the background and experience of Bob Blackledge

Robert D. Blackledge received his BS in chemistry from The Citadel in 1960 and his MS in chemistry from the University of Georgia in 1962. He subsequently worked in forensic science for more than 30 years, including 11 years with the U.S. Army Criminal Investigation Laboratory in Europe. His final position was senior chemist with the Naval Criminal Investigative Service (NCIS) Regional Forensic Laboratory in San Diego, California from 1989 until his retirement in 2006. Blackledge is the author or co-author of approximately 40 articles in journals and chapters in books. His interests are wide-ranging, but his special passion is trace evidence. Reports of his research in this area have been published in the FBI’s Law Enforcement Bulletin; the FBI’s Crime Laboratory Digest; Journal of Forensic Sciences; Science & Justice; Forensic Science International; Forensic Science Review; Microgram Journal; and Analytica Chimica Acta. He is the editor for Forensic Analysis on the Cutting Edge: New Methods for Trace Evidence Analysis (published by Wiley-Interscience in August 2007). He may be reached by e-mail: This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

*For information about the Trace-Evidence Symposium go to

"The Increasingly Diverse and Challenging Discipline of Trace-Evidence Analysis"
November-December 2008 (Volume 6, Number 6)
Evidence Technology Magazine
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