The Evidence is in the Bag
Written by Chris Bily   

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PLASTIC BAGS are one of the most widely distributed and frequently used commercial products in the world. They can be used in a variety of ways, including disposal of waste material, storage of food products, and transportation of consumer goods. While these bags are typically designed to provide consumers with ease and convenience, they are often used by criminals in the course of committing more sinister acts, such as homicide, drug trafficking, robbery, and kidnapping.

When bags are recovered from crime scenes, they are often submitted to the crime laboratory for examination. One of the first considerations is friction-ridge skin impressions. Logic would dictate that if a bag contains some form of physical evidence, then it must have been handled in some manner. Bags can also be examined for other types of evidence, including trace materials and biological evidence. While valuable forensic information can be gleaned by examining the more traditional forms of evidence recovered from a bag, there exists other lesser-known characteristics that are imparted to a bag as a result of the manufacturing process. These characteristics can be instrumental in linking a questioned bag from a crime scene to a known source of bags recovered from a suspect.

Manufacturing process

John Douglas, former head of the Federal Bureau of Investigation’s Investigative Support Unit, once said in regard to violent criminals: “In order to understand the artist you have to look at the painting.” This same type of logic applies to mass-produced items that are of evidential value and forensic interest. In order to understand the potential information contained in a plastic bag, it is imperative to understand how they are manufactured.

The process begins when solid polyethylene pellets and reclaimed plastic are placed into a hopper along with dye pigments. These materials are then melted at temperatures between 400° and 500°F to produce molten plastic. The molten plastic is forced through a screen to remove large impurities such as un-dissolved plastic and pigment granules, and then extruded through a die and mandrel (Figure 1) to form a hollow tube (Figure 2). Another method utilizes a linear die to produce a flat film instead of a tube. This film is then cooled with water, rather than air.

In the tubular blown-film process, the tube is cooled by air blown through the mandrel and die as it travels upward. This cooling process is used to solidify the molten plastic and to produce the desired gauge and size of the polyethylene film product. The cooled plastic is then pulled upward and flattened as it passes through a series of rollers (Figure 3). The film can then be hot-knifed, folded, and heat-sealed to produce a finished product, or it can be spooled onto rolls and stored for later use. Other characteristics may be incorporated into or on the bag such as drawstrings and printed warning information.

Class and subclass characteristics

Class characteristics are properties of evidence that allow like items to be grouped together. In mass-produced items, these are intentional properties that are determined prior to manufacture. With respect to plastic bags, class characteristics consist of construction features such as length, width, thickness, color, weight, fold lines, seal marks, the number and thickness of laminates, hem length (the distance between the edge of the bag and the heat seal), the number of perforations and the distance between them (Figure 4), and type, size, and color of the closure (Figure 5).

According to the Association of Firearm and Tool Mark Examiners (AFTE) Glossary, “subclass characteristics are discernible surface features of an object which are more restrictive than class characteristics in that they are produced incidental to manufacture, are significant in that they relate to a smaller group source, and can arise from a source which changes over time.”

These subclass characteristics are at the heart of the plastic-bag comparison process. When the molten plastic passes through the mandrel and die, undissolved pigment and plastic are deposited on the lips of the die. Certain additives can also cause damage and wear on the die. As the molten plastic passes through the lips of the die, striated tool marks are imparted onto the polyethylene film in the form of extrusion die lines (Figure 6). These striated lines can also occur as a result of poorly dispersed dyes or pigment in the liquid resin mixture that causes the appearance of pigment bands. Die lines and pigment bands are transient in nature and can persist for varying lengths of time. Die lines and pigment bands can be either straight or slanted, depending on the manufacturing process. Slanting lines indicate that the bags are produced on a rotating extruder die. When this process is used, it is possible to reconstruct a sequence of bags.

Another potential source of class, subclass, and individual characteristics is the heat seal. The heat seal is formed when the cooled polyethylene film is pinched between two metal bars. The metal bars are coated with materials such as cloth or Teflon to prevent the film from sticking to it. As the bars are used, they accumulate damage and collect bits of debris. This aggregate of damage and debris creates an impression that persists for a period of time during the production process. A study conducted by the FBI demonstrated that the pattern created by the heat-sealing process can change in as few as 50 bags. Information such as this can allow an examiner to conclude that a questioned bag and a bag from a known source were sealed by the same heat sealer in the same relative period of time. It should be noted that the heat seal examination is conducted independently of the die-line and pigment-band examination. The heat-seal examination is taken into consideration when other characteristics are not present or of insufficient quality for comparison.

Individual characteristics

Individual characteristics are unique to a particular object and separate it from all other objects with like class characteristics. Some familiar examples would be accidental damage to the outsole of footwear, or the striated tool marks in the bore of a firearm.

In regard to plastic bags, individual characteristics exist in the form of fisheyes, arrowheads, and tiger stripes (Figure 8). Fisheyes and arrowheads are the result of undissolved pigment that gets trapped in the polyethylene film. Tiger stripes are caused by vibration and stretching of the film. When all other class and subclass characteristics are in agreement, these individual characteristics can be used to connect a questioned bag with a bag taken from a known source.

Examination and comparison

The comparison process begins with an evaluation of class characteristics. If the class characteristics (for example: length, width, and color) are not in agreement, then the known source of bags can be eliminated as a possible source of the questioned bag. It should be noted that minor discrepancies can and do exist in the characteristics present in a bag—length and width, for example. These minor discrepancies should be taken into consideration before an exclusion can be made.

If class characteristics are found to be in agreement, then the comparison process can continue with the microscopic and visual analysis of subclass and individual characteristics. The questioned bag and the bag from the known source of bags are laid edge-to-edge across a light box and examined for the presence of corresponding die lines, pigment bands, fish eyes, tiger stripes, and arrowheads (Figure 9).

The PRIDE Database

In 2004 the Federal Bureau of Investigation Laboratory Questioned Documents Unit, working in conjunction with the Center for Innovative Technology in Virginia and the Virginia Institute of Forensic Science and Medicine, developed the Polyethylene Repository and Information Database for Evidence (PRIDE). This database was developed in order to provide investigative leads for law enforcement in cases where no exemplar bags are available for comparative purposes. The FBI also maintains a repository of plastic bags that is frequently updated as manufacturers of plastic bags introduce new products.

When a plastic bag of unknown origin is submitted to the Questioned Documents Unit for examination, all discernible class characteristics are analyzed. These class characteristics (for example: color, capacity, number of perforations, and closure type) are entered into the database (Figure 10). The database can then provide a list of potential sources that are individually searched by the examiner (Figure 11). Each individual source contains images of a representative bag and the packaging as well as information regarding manufacturing and retail distribution (Figure 12). The examiner can then visually compare the questioned bag to the image of the bag on the screen. If it is determined that enough similarity exists between the questioned bag and the image on the screen, then a bag can be retrieved from the FBI’s repository of bags and physically compared.

Case study

In April of 1995, the partially clad body of a young girl was found next to a water-pumping station in central Florida. A 30-gallon plastic garbage bag covered her body. After a thorough examination of the victim and crime scene, it was determined that there was a complete absence of any individualizing evidence such as friction-ridge impressions, footwear impressions, or DNA. The only evidence that examiners had to work with was trace evidence in the form of carpet fibers and animal hairs—and the garbage bag.

A man who was known to have frequent contact with the victim was developed as a suspect. In addition to fiber and animal hair samples, a box of garbage bags was seized from his residence. The bags were consistent in construction characteristics with the bag covering the victim. The challenge became associating the bag from the victim to the box of bags found in the suspect’s residence.

The examiner made a trip to the plant where the bags were manufactured. Here, he was able to view the manufacturing process and acquire samples at different intervals during the production process. By analyzing the persistence of the die lines and pigment bands on the samples taken over time, acquiring retailer distribution information, and calculating an approximate number of boxes of bags that would have die lines and pigment bands similar to the questioned bag, the examiner was able to provide extremely compelling evidence.

When faced with the mounting circumstantial evidence, the suspect pled guilty to second degree murder. He was ultimately sentenced to a 40-year prison term with no possibility of parole. Because of the tenacity of the examiner and his willingness to go above and beyond, the victim and her family were able to obtain some measure of justice. This case was featured on an episode of Forensic Files titled “A Bag of Evidence”.


This article was not written to provide all-encompassing knowledge about the forensic examination of plastic bags. It does, however, provide some fundamental information about the plastic-bag manufacturing process, the forensically significant information imparted into and on a bag as a result of the manufacturing process, and what can be learned from the examination of this information. For more information about polyethylene film comparisons or the PRIDE database, please contact the FBI Laboratory Questioned Documents Unit: 703-632-8444


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Figure 12


The author would like to thank Lorie Gottesman of the FBI’s Questioned Document Unit at the FBI Laboratory and Ted Schwartz, a trace evidence examiner with the Westchester County (New York) Forensic Laboratory, for peer reviewing the article and providing images. Thanks also to Dr. Keith Morris of the West Virginia University Forensic and Investigative Sciences Program for providing assistance in taking photographs. Finally, the author would like to express appreciation to Brandon Smithson, graphic designer, for lending his talents to this project and to the rest of the West Virginia University Forensic Science Initiative staff for supplying garbage bags to work with.

About the Author

Chris Bily is a forensic science curriculum designer with the West Virginia University Forensic Science Initiative. He specializes in developing training and educational materials in the areas of friction ridge skin identification and firearm and tool mark identification. For more information about the West Virginia Forensic Science Initiative visit:
Bily can be reached via e-mail at: This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

"The Evidence is in the Bag," written by Chris Bily
July-August 2010 (Volume 8, Number 4)
Evidence Technology Magazine
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