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Research and Technology - Forensic Science Communications - July 2008

Research and Technology - Forensic Science Communications - July 2008

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July 2008 - Volume 10 - Number 3

 

Research and Technology

Persistence of Fibers on Ski Masks During Transit and Processing

Deborah D. Chewning
Physical Scientist
Trace Evidence Unit
FBI Laboratory
Quantico, Virginia

Kc L. Deaver
Evidence Analyst
Evidence Control Unit
FBI Laboratory
Quantico, Virginia

Angi M. Christensen
Physical Scientist
Trace Evidence Unit
FBI Laboratory
Quantico, Virginia

Abstract | Introduction | Materials and Methods | Results | Discussion and Conclusions | Acknowledgments | References

Abstract

This study investigates the persistence of fibers on the inside and outside surfaces of ski masks during transit to the FBI Laboratory and during evidence processing to see if separate examinations of the inside and outside of a mask are valuable and warranted. Twenty ski masks were seeded with 50 test fibers each on either the inside or outside only. The masks were then packaged, shipped, and processed according to protocol, and the final recovery location of the fibers was documented. Results indicated that 11 (55%) of the ski masks showed evidence of test-fiber transfer sometime during the study, although the number of transferred fibers was rather small, ranging from one to three. Although the probability that a fiber will be recovered from the same side on which it was deposited is higher than the probability that it will be recovered from the opposite side, an examiner cannot conclude that fibers recovered in the laboratory from the inside (or outside) of an item were originally deposited on that side. In many cases, it may be adequate to process all surfaces of ski masks or other head coverings together.

Introduction

Locard’s Exchange Principle, which states that whenever two objects come into physical contact, an exchange of materials takes place (Locard 1920; Locard 1929), forms the foundation of trace evidence examination. Trace material can take such forms as textile fibers from clothing, hairs from different body areas, and chips of nail polish. By examining evidentiary items for trace evidence, it may be possible to associate two people or a person and an object involved in a crime.

The caseload of the FBI Laboratory’s Trace Evidence Unit is highly varied and includes robberies, rapes, and terrorist activities. These crimes often are committed by perpetrators who wear head and/or facial coverings in order to conceal their identities. Knit pullover ski masks are common coverings. When a ski mask is recovered, it is submitted to the FBI Laboratory for various analyses, including hair and fiber examinations.

During a hair and fiber examination, evidence is processed to locate and recover potential hairs and fibers. Processing textile items in the FBI Laboratory most often involves the combination of scraping and picking techniques. For ski masks and other head or facial coverings, the debris from the inside of an item usually is collected and examined separately from the debris from the outside of the item. The reason for this is the notion that hairs recovered from the inside of an item are more likely to have originated from the person who was wearing it (assuming that the item was worn as intended). The aim of this study was to determine whether trace evidence, including hairs and fibers, transfers between the inside and outside of an item during transit to the Laboratory or during evidence processing or both. Specifically, we examined the transfer and persistence of fibers on ski masks to see if separate examinations of the inside and outside are valuable and warranted.

Materials and Methods

Twenty black ski masks (100 percent acrylic, three-hole ski mask; purchased from Terence Int’l, doing business as TansClub.com; Los Angeles, California) were purchased new and were not washed. Carpet fibers of approximately one-quarter to one-half inch in length were counted into groups of 50 and planted on the ski masks, which were assigned identification numbers 1 through 20. Ski masks 1 through 10 were seeded with test fibers on the inside only, and ski masks 11 through 20 were seeded on the outside only. To seed the outside surface, the fibers were sprinkled as evenly as possible from a paper fold, and then a clean sheet of paper was pressed firmly onto the surface of the mask to facilitate the fibers’ adherence to the mask. To seed the inside, the mask was held open, the fibers were sprinkled by hand, and the mask was closed and pressed firmly against itself.

The fiber type was selected because it fluoresces at 475 nm using an alternate light source (ALS) (CrimeScope CS-16-400, SPEX Forensics, Edison, New Jersey) (Figure 1), making it easy to locate and identify when seeded on the black ski masks, whose fibers did not fluoresce. In a manner similar to that used by evidence recovery teams at a crime scene, each ski mask was packaged in a tape-sealed Tyvek envelope and labeled with its identification number. Envelopes were packaged in pairs in medium FedEx shipping boxes and shipped via FedEx from the FBI Laboratory to the home address of one of the authors (DDC).

An image of a fluorescing test fiber on a ski mask

Figure 1: Fluorescing test fiber on a ski mask

The boxes containing the masks were transported via car to the Laboratory by DDC. The boxes were taken individually into a clean processing room where two of the authors (DDC, KLD) opened the packaging and performed a preprocessing screen using ALS to see if any of the fibers had already transferred. Prior to any processing, the masks were examined using the ALS at 475 nm with an orange filter (Figure 2). Because the selected seeding fibers fluoresced, the location of the fibers could quickly and easily be determined. Any transfer of test fibers from their original location was noted.

A photograph showing an examiner performing a preprocessing screen using an alternate light source

Figure 2: Preprocessing screen using an alternate light source

Immediately following the screening with the ALS, the processing was begun with a clean laboratory coat and gloves, sanitized work area, and a clean piece of paper on the processing table. The ski masks were processed by one of the authors (AMC) according to FBI Laboratory protocol, which involves using a scraping technique (Figure 3). The outside of the mask was processed first, and any collected fibers were transferred to a pillbox and labeled with the mask’s identification number and location of collection (inside or outside). A new sheet of brown paper was placed down, and laboratory coat and gloves were changed. The inside of the ski mask was then processed using the same technique. Between each ski mask, the work area and instruments were sanitized, and new paper, laboratory coat, and gloves were used.

A photograph showing an examiner processing a ski mask using the scraping technique

Figure 3: Processing a ski mask using a scraping technique

When all of the processing was completed, the pillboxes containing the collected fibers were examined using the CrimeScope at the same wavelength. The total number of seed fibers recovered in each pillbox was counted and documented.

Results

Eleven (55%) of the ski masks showed evidence of test-fiber transfer sometime during the study (Table 1). For five (25%) of the masks, transfer was noted during the preprocessing screen, indicating that the transfer occurred during transit. Table 1 indicates the number of fibers recovered from each item during processing and the number and percentage of recovered fibers from the nonseeded side of the item. For masks seeded on the inside, the total number of fibers recovered from their original surface (the inside) ranged from 21 to 44 of the original 50. For masks seeded on the outside, the total number of fibers recovered from the outside ranged from 22 to 38 of 50. The majority of the fibers were recovered from their original surface location, but some were not recovered at all, and others exhibited transfer from one surface to the other (see Table 1). Transfer may have occurred during packaging or shipping or during the scraping and collection procedures. When transfer did occur, the number of transferred fibers was relatively small, ranging from one to three fibers. No attempt was made to locate unrecovered fibers, and unrecovered fibers were not used during any of the calculations. These fibers may have been lost during the seeding procedure, become dislodged from the ski masks during or prior to packaging, remained within the Tyvek envelopes, or been lost to the processing environment during scraping.

Table 1: Transfer of Seeded Test Fibers

Item Number

Total Fibers Recovered
(out of 50)

Fibers Recovered from Nonseeded Side (%)
1 (seeded inside)     
36
            0 (0)
2 (seeded inside)     
21
            0 (0)
3 (seeded inside)     
36
            2 (5.5)
4 (seeded inside)     
35
            2 (5.7)
5 (seeded inside)     
44
            3 (6.8)
6 (seeded inside)     
37
            1 (2.7)
7 (seeded inside)     
31
            0 (0)
8 (seeded inside)     
37
            0 (0)
9 (seeded inside)     
39
            0 (0)
10 (seeded inside)     
38
            0 (0)
11 (seeded outside)   
34
            1 (2.9)
12 (seeded outside)   
34
            1 (2.9)
13 (seeded outside)   
38
            1 (2.6)
14 (seeded outside)   
35
            1 (2.9)
15 (seeded outside)   
35
            0 (0)
16 (seeded outside)   
37
            1 (2.7)
17 (seeded outside)   
22
            2 (9.1)
18 (seeded outside)   
29
            2 (6.9)
19 (seeded outside)   
32
            0 (0)
20 (seeded outside)   
27
            0 (0)
Total Items Exhibiting Fiber Transfer
11 of 20 (55%)


Discussion and Conclusions

In cases where evidence requires multiple types of examinations, the FBI Laboratory’s Trace Evidence Unit is usually the first unit to receive evidence. Technicians and examiners must work quickly to ensure effective and efficient evidence flow through the Laboratory in order to meet deadlines such as trial dates. Processing and examining the inside and outside surfaces of an item of evidence separately take nearly twice as long as processing the total surface area collectively. This study showed varied persistence of certain carpet fibers on one surface of a ski mask between the crime scene and the Laboratory examiner. Because the majority of fibers were recovered from their original seeded location (i.e., did not transfer), there is a relatively high probability that fibers recovered from a location originated from that location. Because, however, more than half of the specimens displayed some fiber transfer between surfaces, an examiner cannot be sure that fibers recovered from the inside (or outside) of an item were actually deposited on that side originally.

Trace evidence examiners should be careful making statements regarding the specific source of recovered trace evidence, and it may be unwise to state that a questioned fiber originated from or was originally deposited on the inside or outside of a submitted item. At the same time, our results indicate that it may be adequate to collect all trace evidence from ski masks or other head coverings together. Additional research that includes hairs and other types of fibers is needed, as their different lengths, coarseness, texture, entanglement in the garment, and other physical characteristics may affect their likelihood of transfer.

Acknowledgments

This paper was originally presented at the Trace Evidence Symposium, sponsored by the National Institute of Justice and the FBI Laboratory, August 13–16, 2007. It was edited for publication. It can be found in its original form at http://projects.nfstc.org/trace/docs/.

This is publication number 08-12 of the Laboratory Division of the Federal Bureau of Investigation. Names of commercial manufacturers are provided for identification only, and inclusion does not imply endorsement by the FBI.

Author Deborah D. Chewning recently transferred to the FBI Laboratory’s Cryptanalysis and Racketeering Records Unit. 

References

Locard, E. L’enquete criminelle et les methodes scientifiques. Flammarion, Paris, France, 1920.

Locard, E. L’analyse des poussieres en criminalistique, Revue Internationale de Criminalistique. (September 1929) 176–249.