Research and Technology - Forensic Science Communications - July 2008

Research and Technology - Forensic Science Communications - July 2008


July 2008 - Volume 10 - Number 3


Research and Technology

PRIDE: The Polyethylene Repository and Information Database for Evidence

Lorie L. Gottesman
Forensic Document Examiner
Questioned Documents Unit
FBI Laboratory
Quantico, Virginia

Plastic bags, or polyethylene film products, are often used in a variety of crimes—including homicides, kidnappings, bank robberies, and drug offenses—and each plastic bag may provide clues to its origin or manufacturing source. Polyethylene film products are produced by feeding resin pellets into a hopper. The pellets are melted and extruded through a screen and then between a mandrel and a ring-shaped die. The film is manufactured either in a tubular fashion or by a flat casting method.

In the tubular method, air is blown on both the inner and outer sides of the tube as the molten plastic is emitted from the extruder. The tube is expanded to the desired size by the air emitted from the mandrel (Figure 1). After the tube is drawn upward, often several stories high (Figure 2), it is flattened between a series of rollers (Figure 3). It is subsequently cut, possibly printed with warning or write-on labels, perforated, folded, heat-sealed or hot-knifed, and packaged.

Figure 1: A circular die used for manufacturing plastic bags. The air emitted from the mandrel expands the plastic to the appropriate size and shape.

Figure 2: A plastic column reaching several stories high will be transformed into a bag.

Figure 3: A series of rollers flattens the plastic and transports it through the manufacturing process.

In the casting method, instead of inflating the tube, the film is extruded through a linear die and the film remains flat. The plastic is cooled by water, producing one flat sheet of plastic. This method is used for small bags, because the flat film can be halved and cut into smaller sections. This method is often used for products that include postapplied additions to the film, such as drawstrings and zippers, because the plastic can be easily folded over the additions and/or heat-sealed to complete the bag (Figure 4).

Figure 4: A plastic bag with a drawstring closure

Examiners of polyethylene film must evaluate numerous possible characteristics when determining whether a bag found at a crime scene was attached at one time to a bag of known origin or to another questioned bag from another crime scene. The difficulty arises when a sole questioned bag is collected and the field investigator is interested in the type of bag, its manufacturer, and any other information the FBI Laboratory can glean from it. Unfortunately, in the past, particularly with only one evidentiary plastic bag and no known bags for comparison, the Laboratory had been unable to provide any pertinent information to assist investigators.

In 2004, after numerous requests from law enforcement personnel needing to know the origin of plastic bags collected as evidence, personnel in the FBI Laboratory’s Questioned Documents Unit (QDU) initiated a research project involving plastic bags and their characteristics. The resulting database, the Polyethylene Repository and Information Database for Evidence (PRIDE), enables the QDU to assist law enforcement in the fight against crime by providing timely information on the origin of evidentiary plastic bags. Developed by the Center for Innovative Technology in Herndon, Virginia; the Virginia Institute of Forensic Science and Medicine in Richmond, Virginia; and the FBI Laboratory, the PRIDE contains information on the attributes, manufacturer, and retailers for each bag, as well as a repository of sample bags.

When a case submitted to the QDU requires a PRIDE search, the questioned bag is first categorized based on its intended use—disposal, storage, freezer, sandwich, snack, or other. The next category is the closure type, which includes reclosable, drawstring, flap, fold-top, handle, bar-seal, slider, strap, and twist-tie. If present, closures are further categorized based on their color(s). The bag is then categorized by its capacity, if known. If unknown, an estimate may be made. Bag color is the next area of categorization, followed by whether the bag is scented or unscented (Figure 5).

Figure 5: A screen shot of the PRIDE database showing the search screen and the characteristics (as described in the text) entered to run a sample search. The results of the search are provided in Figures 7–13.

Other physical characteristics—including seals and perforations, write-on and warning labels, gussets, gloss, clarity, extrusion lines, and package patterns—are examined and categorized. Many of these areas can be subdivided further. For example, perforations can be further classified based on the cut design and the distance between perforations. When classifying the warning label, the examiner can add text as well as any codes in the text and whether the code is repeating. In addition, the system also contains images of warning-label text (Figure 6). Using this text, an unknown bag can be associated with a bag already in the database.

Figure 6: The warning label on a plastic bag, reminding users to keep the bag away from children to avoid suffocation

The examiner conducts a search by entering characteristics (whether a few or numerous) until a reasonable number of potential associations are obtained. Once this occurs, the bags are manually searched for a possible match. The PRIDE contains images of the bags and packaging, as well as information on the physical characteristics, distributor, and manufacturer (Figures 7–13). In addition to database information, the PRIDE includes a repository of sample polyethylene bags that correspond with the sample data. Therefore, if necessary, not only a virtual comparison but also a physical comparison can be conducted on the plastic bags.

Figure 7: The results of the search conducted using the characteristics entered into the PRIDE and displayed in Figure 5. The details of the bag, as well as images of the product, are displayed on the screen. From this screen, the user can view additional details (see Figure 8), update bag information, or delete the record entirely.

Figure 8: The basic physical characteristics—including size, color, and type—of the plastic bag returned in the search conducted using the information shown in Figure 5. This bag is a Ruffies 39-gallon lawn and leaf bag with a drawstring closure.

Figure 9: More specific details, including actual measurements, and a photograph of the bag

Figure 10: The measurements noted on the bag and its closure information

Figure 11: Data on the seal and perforations of the bag

Figure 12: Warning-label information and a photograph, as well as manufacturer information, of the bag

Figure 13: A local retail establishment that sells the bag

The PRIDE will evolve and expand as the polyethylene industry changes and introduces new products. The user-friendly design of the database allows QDU personnel to easily enter and scan new products (Figure 14). Personnel from the QDU will purchase new products, enter the information, and store the sample bags in the repository for future reference.

Figure 14: The characteristics of a new plastic bag being entered into the PRIDE

Questions concerning the PRIDE or polyethylene film examinations may be directed to the FBI Laboratory Questioned Documents Unit at 703-632-8444.


This is a revised version of an article that previously appeared in the Fall 2007 issue of the Chesapeake Examiner, the official publication of the Chesapeake Bay Division of the International Association for Identification. It was used with the permission of the editor.

This is publication number 08-06 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.