Fibers – Trace Evidence for solving Crime

Locard’s Principle states that “Every contact leaves a trace.” (McDermid, 2014). We all understand this means that a criminal will leave traces of their presence at a crime scene, but it also means that the crime scene will leave traces on the criminal. One of the most common forms of trace evidence used for solving crime is fibers (Simplified Guide, Introduction, 2013). By itself, fiber corroboration is not enough to convict a person of a crime (Trace Evidence) but, together with other confirmation, they can link a suspect to a scene. With this in mind, finding and matching these minute links between the scene and the suspect will help the police in their investigations.

What are Fibers?

Fibers are defined as small pieces of fabric, longer than they are wide; they can be natural or man-made and have a lot in common with hair (Deedrick, 2000).  Natural fibers each have their distinctive patterns as seen in the cover image for this post. A skilled analyst should recognize them by sight. Synthetic fibers are smoother. Each type is created by squeezing the polymer through a small hole to create a strand. Each batch of synthetic fiber will have consistent color and diameters, but there is a wide variety between types and batches. This allows identification and possible matching (Trace Evidence).

Multiple fibers are spun together into a yarn, and yarns are knitted or woven into fabrics and clothing. Clothes and fabrics shed fibers at a rate which depends on how tightly they are spun and woven together.

Most often fibers at a scene are invisible to the naked eye. When these microscopic pieces of evidence transfer at a crime scene, the Crime Scene Investigators (CSIs) work to find them.

How is this ‘almost invisible’ evidence collected

As the CSIs go over a crime they attempt to identify the most likely places to find fibers. In a murder, around and under the body is an obvious place. The victim’s clothing is another probable place to find strands of evidence.

When examining clothing the easiest way to discover evidence is by brushing it onto a clean, sterile piece of paper. The paper is carefully folded around the findings and stored in a labelled evidence bag.

Image 1: Evidence bag credit: https://study.com/academy/lesson/recording-preserving-evidence-methods-procedures.html

When searching around the body, it gets more difficult. The CSIs can vacuum a carpet. Or they can examine the floor closely and pick up possible evidence with tweezers.

Also, there is taping—which is just how it sounds. Sticky tape is laid on the surface and it picks up any loose items. It is efficient, as the clearly labelled tape can be taken to the laboratory and examined under a macroscope. The problem with taping is that the adhesive can destroy evidence as it is transferred to slides for microscopy (Simplified Guide, Trace Evidence, 2013).

How are fibers matched?

In the first instance, fibers are observed under a microscope. If there is a reference fiber, such as a woollen sweater from a suspect, this can be analysed alongside our evidence.

Microscopy

Imagine the reference is a green sweater. An adhesive tapelift was taken at the crime scene. Examination of the tape shows several green fibers. These are transferred to microscope slides, along with fibers from the reference. Using a comparison microscope, an expert can categorize the unique structures of each piece of evidence, and eliminate all the non-wool fibers (Fiber, 2017). Now we have to look at other possibilities. Color is very subjective; what one person thinks of as teal, another person might see as turquoise. However, just by eye, we can tell the difference between a blue-green fiber and a yellow-green fiber. So if they don’t match our green reference fiber they can be put aside. Leaving the rest of the samples which appear similar to the reference. Matching by eye will not stand up in court, and the whole purpose of Forensic Science is the application of Science to the Law. If it is not court-worthy evidence, then further tests are necessary.

Microspectrophotometry

This is a specialist microscope, which enables the forensic scientist to examine the colors of fiber. It measures the reflective and transmission effects under differing wavelengths of visible light. The technique is discriminating and can establish a difference between fibers that appear to be the same color. Also, it is a non-destructive test that is fast and reliable. When hooked up to a computer, it charts the reflectivity and transmission and can overlay the graphs for comparison (Can Hu, 2020).

Thin Layer Chromatography

Not only is this older method useful in determining the difference between pen inks ( for an explanation of how TLC works see article here, Thin Layer Chromatography (TLC) is also useful in identifying whether two fibers are colored using the same dye batch. Different manufacturers will use different combinations of dyes to produce almost the same color (indistinguishable by eye). If the dye from our hypothetical green sweater yields a different pattern on a TLC plate to that of our evidence, then there is no match (Wiggins, 2005).

Fourier Transform Infra-Red

Fourier Transform Infra-Red (FTIR) is particularly useful for the analysis of fibers. Under a microscope, many fibers look smooth and it is difficult to tell nylon from polyester.

Image 2 FTIR Spectra
Credit: https://kaplanscientific.nl/wp-content/uploads/2020/01/image-1024×576.png

With  FTIR the difference stands out in the spectra as seen in the image above. Additionally, the technique identifies compounds added to the fabric, such as flame retardant chemicals (Analyzing, 2020).

The important part of scientific analysis of evidence is to continue through all the techniques until a difference is identified. If all the techniques fail to find a difference then we can say they are matched.

How likely is a coincidental match between fibers

A coincidental match between two fibers is something that needs to be ruled out. If you think about it, the possibility of a match between two fibers that have nothing to do with the crime scene would depend on their frequency in the environment. A team took tapelifts of bus seats, pub seats and cinema seats and set about cataloging the numbers of fibers. They took two reference fibers and tapelifts from seats in buses, public houses and cinemas. Using comparison microscopy, they found no matches with their reference fibers (Palmer, 2015).

Image 3 Fiber Frequency
Credit: Palmer (2009)(https://doi.org/10.1016/j.scijus.2009.02.008.

In Image 3 from a different study, the researchers found that Black or grey cotton was the most often found in the environment closely followed by blue cotton (Palmer, 2009).

An experiment was conducted by taking samples from three types of different crime scenes: bank robbery, kidnapping and murder. Using the methods of analysis mentioned above, the team compared 2083 different fibers. None of them showed any matching characteristics (Houck, 2003).

Maybe the person visited the scene before or after the crime

How long does trace evidence stay on a person or their clothing?  Many factors influence the persistence of fibers on a person. As with glass fragments,(mentioned in a previous article see here), the type of clothing and whether the suspect walks or runs away from the scene will influence how long trace evidence stays on a person. Using particles that glow under UV light sources, researchers learned how long these specks stay on a person under a variety of conditions. From this, a mathematical formula was developed to work out the probability of how long trace evidence stays on a person or clothing (Ménard, 2021). (If you are interested in complex, probability mathematics please check the reference, I found it fascinating.)

So a suspect insists they visited last week, and the crime was yesterday. Several fibers are taken from their clothing which links them to the crime scene. By counting the fibers on the suspect, the probability of persistence can be calculated. In this way, scientists can mathematically estimate when the contact to transfer the fibers occurred. A large number of fibers suggests a more recent exposure.

Conclusion

When CSIs and forensic scientists work to recover fibers they look at the possibility of contact between a suspect and the crime scene. Using tested scientific methods, fibers found on a suspect match those at the crime scene can link the two. Mathematics can estimate how long ago the contact took place, establishing a time frame.

While fibers alone cannot convict, if the crime took place within that time frame then is another building block of evidence to help police solve a crime.

References

Analyzing Micro-Fibers Using FTIR Microscopy. (2020). Kaplan Scientific. https://kaplanscientific.nl/evaluation-of-micro-fibers-utilizing-microspectroscopy/

A Simplified Guide to Forensic Science. (2013). Introduction. NFSTC. https://www.forensicsciencesimplified.org/trace/#:~:text=Trace%20evidence%20can%20include%20a,soil%2C%20cosmetics%20and%20fire%20debris

A Simplified Guide to Forensic Science. (2013). Trace Evidence. NFSTC. https://www.forensicsciencesimplified.org/trace/how.html#:~:text=Common%20collection%20methods%20include%20individual,the%20destructive%20nature%20of%20adhesives

Can Hu, Hongcheng Mei, Hongling Guo, Jun Zhu. (2020) Color analysis of textile fibers by Microspectrophotometry. Forensic Chemistry. 18. https://doi.org/10.1016/j.forc.2020.100221 .

Deedrick, D.W. (2000) Hairs, Fibers, Crime, and Evidence Part 2: Fiber Evidence. Forensic Science Communications. Vol2(3). https://archives.fbi.gov/archives/about-us/lab/forensic-science-communications/fsc/july2000/deedric3.htm#:~:text=As%20discussed%20previously%2C%20fibers%20are,furniture%20at%20a%20crime%20scene

Fiber Microscope Test? (2017). Textile Engineering. http://textilecreativities.blogspot.com/2017/12/fiber-microscopic-test.html

Houck M. M. (2003). Inter-comparison of unrelated fiber evidence. Forensic science international, 135(2), 146–149. https://doi.org/10.1016/s0379-0738(03)00195-6

McDermid, V. (2014). Forensics: The Anatomy of Crime. Profile Books LTD.

Ménard, H., Cole, C., Mudie, R.,  Klu, J.K., Lawson, M., Green, S., Doyle, S., MacNeill, E.H., Hamilton, B., Sheridan, K., & Daéid, N.N. (2021). Creation of a universal experimental protocol for the investigation of transfer and persistence of trace evidence: Part 2 – Implementation and preliminary data. Forensic Science International: Synergy. Volume 3. https://doi.org/10.1016/j.fsisyn.2021.100164.

Palmer, R., Burch, H.J., (2009). The population, transfer and persistence of fibers on the skin of living subjects. Science & Justice. Volume 49, Issue 4. https://doi.org/10.1016/j.scijus.2009.02.008.

Palmer, R., Burnett, E., Luff, N., Wagner, C., Stinga, G., Carney, C., & Sheridan, K. (2015) The prevalence of two ‘commonly’ encountered synthetic target fibers within a large urban environment. Science & Justice. Volume 55, Issue 2, https://doi.org/10.1016/j.scijus.2015.01.001.

Trace Evidence. The Forensics Library. https://aboutforensics.co.uk/trace-evidence/

Wiggins, K. G., Holness, J. A., & March, B. M. (2005). The importance of thin layer chromatography and UV microspectrophotometry in the analysis of reactive dyes released from wool and cotton fibers. Journal of forensic sciences, 50(2), 364–368.