Textile Forensics: How Small Clues Can Crack a Case

Forensic scientists are a bold breed. Who else could, when asked to determine the makeup and origins of a single stray thread, accept the task with zeal? Welcome to the world of textile forensics, which analyzes fibers and fabrics found at crime scenes. Identifying those fibers and fabrics, and tracing them back to their origins, can, in turn, link them with suspects, victims, and locations. Entire investigations might hinge upon analyzing a sample thinner than a strand of human hair.

Fibers and fabrics are everywhere, but each individual piece has its own unique journey. The complexities of international commerce mean that fiber and fabric work their way through byzantine supply chains before ending up at crime scenes. Samples from recycled or repurposed items could be on their second or third life. Forensic scientists who specialize in textile materials need to be as attuned to these complexities as they are to the science of laboratory analysis.

Read on to learn more about textile forensics and how it’s used to serve justice.

Meet the Expert: Max Houck, PhD

An award-winning international expert in the forensic sciences, Dr. Max M. Houck has nearly 30 years of expertise in casework, research, management, and writing. His casework includes the Branch Davidian Investigation, the September 11 attacks on the Pentagon, the D.B. Cooper case, and the West Memphis Three, among others. He has managed tens of millions of dollars in grants and his committee work includes the White House, the National Academies of Science, the Royal Society, and Interpol.

Dr. Houck is one of the most published professionals in his field, with over 20 books and dozens of peer-reviewed articles. His research has ranged from microscopic trace evidence to how forensic service providers operate as systems.

In addition to chairing the Forensic Science Educational Program Accreditation Commission for six years, Dr. Houck has helped or started forensic academic programs at West Virginia University, the University of Southern Florida, and Florida International University. His Substack, Forensic Science*, explores forensic science and the role it plays in criminal justice.

How Forensic Scientists Work with Textiles

“The first thing you do is you just look at it,” Dr. Houck says. “Usually, that’s done by using a transmitted-light microscope. You mount your sample on a slide, and you look at its physical characteristics. What’s the color? What’s its cross-sectional shape? There are probably 400 to 500 different cross-sectional shapes that are in use at any one time.”

Textile materials are different from latent prints and DNA in that there’s no database to compare samples to on a one-to-one basis. Instead, forensic scientists working with textile material will mark down the shape they see. Typical shapes can include trilobe, three-fin, octagonal, dog-bone, and many others. After noting the shape, scientists will look at the sample under different varieties of light, the first being polarized light. That allows them to determine the optical characteristics of the fiber, which aids identification.

“Think of it like the difference between oak and birch and pine,” Dr. Houck says. “You’re looking at how these things are organized internally.”

Optical properties help discern a sample from other polymer types. For example, even if you have two nylon fibers, and even if they have the same cross-section, their optical properties might be different if they were made differently. When polymers are made—in a process called ‘spinning dope’—it’s like honey getting pulled through a showerhead. And the holes in that showerhead (which is actually a spinneret) determine what the final cross-sectional shape of the fibers are. But once they come out, they get gathered up and pulled at a certain rate, which gives them a particular density. So fiber density can vary from batch to batch.

“You could have two fibers with the same diameter and the same cross-sectional shape,” Dr. Houck says. “But if they were pulled at different rates, then those internal optical properties would allow you to know that, and know that they came from two different batches.”

Instrumentation in Textile Forensics

After identifying the sample and noting its optical properties, the next step is verifying the sample’s polymer type. This is done via instrumentation, typically with an infrared spectrometer, the most common type of which is called a Fourier Transform Infrared (FT-IR) Spectrometer. This instrument hits the fiber sample with the beam of infrared light and it makes the fiber vibrate; the type and orientation of each of the chemical bonds in that fiber will vibrate differently as a result. Based on those vibrations, the instrument can determine the type of polymer in the sample.

“When I’m testifying, there’s a metaphor I use for this,” Dr. Houck says. “If I took a basketball and I threw it into an orchestra pit, I could tell you if there was a harp or a banjo or a ukulele or violin down there based on the vibration of the strings. This is the same idea: you’re making the molecules vibrate, and your instrument is listening.”

Once a forensic scientist has verified their polymer in this way, they’ll still need to verify its subtype (there’s more than one type of nylon, for example). Then they’ll verify its color, which is done with a microspectrophotometer. The microspectrophotometer shoots a beam of UV visible light across the spectrum into the fiber and determines its color. Because dye uptake into fiber is not always uniform, the forensic scientist will usually take multiple shots along the fiber to account for any variation, then plot the results on a graph to see if the colors line up.

Going beyond color, there are additional tests that can be performed, such as dye analysis. However, those are rare, as they are very hands-on, time-consuming, and resource-intensive.

“For how small they are, fibers are really complicated things,” Dr. Houck says.

Moving Between Analysis and Investigation in Textile Forensics

Okay, so now what? Knowing the color, shape, and polymer of a piece of textile material isn’t, in itself, enough to tell you where it came from. In fact, those characteristics, without corroborating context, can be misleading. This is where the other side of textile forensics comes into play: investigating the origins of a sample based on what’s been learned in the lab.

“It comes down to an individual analysts looking at the evidence and going, ‘Is there something else here?’” Dr. Houck says.

Whether a forensic scientist gets involved in this sleuthing aspect of the case depends on a few different factors. How big of a case is it? How specific is the evidence? And how crucial of a component is the origin of this sample? And not every analyst knows how to connect a piece of textile evidence with the industry that created it.

“The connection with industry is something we’re losing in forensic science,” Dr. Houck says. “People don’t appreciate the complexity of the supply chain. They don’t know where to start.”

Tracing a sample’s movement from origin to crime scene is no simple task. The knowledge of where a particular fiber came from may rest with a single employee of a rope and cordage institute in New Hampshire, for example, or on a paper printout in the back of a dusty file cabinet at some obscure manufacturing association. Not everyone has the wherewithal, let alone the bandwidth, to do that level of investigation today.

“The number of units in laboratories that do fiber exams is probably decreasing,” Dr. Houck says. “It’s harder to teach academically because it’s so involved. And with the pressures of casework, I think it’s becoming more and more rare to really do the sleuthing part.”

The Impact of Supply Chains on Textile Forensics

The intricacies of modern, international supply chains mean that a textile sample often has taken a long and circuitous path before arriving at a crime scene. As Dr. Houck writes in his blog, Forensic Science*, recycled items can give textile materials a second or even third life. The fibers in the center of a thick rope might be repurposed from carpet. A shirt might be unwoven and rewoven in a factory in Pakistan. Even the labeling on a specific item isn’t necessarily definitive, given the cut corners of global commerce. Investigators need to be aware of these variables, or else they might arrive at an incorrect conclusion.

“There are companies that used to take old blue jeans and shred them up and use them as insulation and in buildings,” Dr. Houck says. “Well, in a shooting reconstruction, if a bullet goes through an insulated wall, you might have fibers introduced that weren’t actually in the room environment. You really need to be aware of this sort of thing.”

Case Example: The Golden (Brown-Green) Thread

Dr. Houck relates a story about a case he was assigned while at the Bureau, where a woman had been found deceased on the side of a road. The only physical evidence collected was a small fiber found during the autopsy. The fiber, about the width of a pinky nail, was the only clue available.

The fiber measured 45 microns in diameter, which is relatively large and suggested it was likely from a carpet. Dr. Houck identified the fiber as rayon, a material made from regenerated wood pulp and cotton waste. But rayon was not typically used in carpets due to its lack of durability. The unusual color of the fiber, described as an unattractive brownish-green, was another key detail.

“You wouldn’t want to own anything this color unless you were in the Army,” Dr. Houck laughs.

After extensive research and phone calls, Dr. Houck learned that a particular model of General Motors cars from the late 1970s used a mix of nylon and rayon in their carpeting—and in this color. Further investigation revealed that only eight cars of that specific make and model from the relevant time period were registered in the five-state area surrounding the discovery site. A subsequent query to the local investigator revealed that one of the suspects owned an older car matching the description, including the unusual carpet color. This crucial evidence eventually led to a suspect’s confession.

Notably, the magic of this case was not found in high-powered technology or rigorous science, though both did play a part. Instead, what cracked it open was two weeks of phone calls, asking questions, getting passed on from person to person, navigating dead ends—people who’d retired, records that’d been lost, documents that’d gone missing—and, most importantly, not giving up.

“It took a lot of patience,” Dr. Houck says. “There were a lot of dead ends. I needed a combination of knowledge and luck. But justice was served.”