DNA, Latent Prints, and Contamination
Written by John Louis Larsen   

THE PURPOSE OF THIS ARTICLE is to clarify practical evidence collection guidelines for the recovery of latent fingerprint and DNA evidence in the field. It is the goal of all of law enforcement to efficiently and effectively collect and preserve evidence for trial. Physical evidence generates the foundation upon which an affirmative prosecution can be generated.

This article appeared in the July-August 2020 issue of Evidence Technology Magazine.
You can view that issue here.

First, I want to clarify a number of misnomers pertaining to whether or not cyanoacrylate (hereafter superglue fuming or “fuming”) destroys DNA. Supergluing does not destroy DNA. The fuming process conducted in the laboratory in a fuming cabinet or a vacuum chamber does not alter the DNA. According to Dr. Karl Reich of Independent Forensics of Illinois1 the fuming process forms a thin “molecular size” shell over the biological material. For example, latent fingerprint impressions affixed to the object being fumed are covered by this thin molecular shell. The beauty in locating a print smudge or smear is that this provides the crime scene technician with an area that can be swabbed for DNA. This unearthed print is called a “latent impression,” meaning hidden—“that is, the print many times is not readily visible.”2 The latent impression is manufactured by the transfer of bodily materials—such as oils from the scalp, hair, or moisture (sweat)—collecting on the ridges of the impression.

The fuming process of items for latent impressions, both in the field and laboratory, will not hinder the collection of DNA as long as the fingerprint powders and applicators are free of DNA which could contaminate and skew the findings. The shell-like structure masking the DNA can be dissolved with acetone. Laboratory processing, breaking down, and clearing the site of the film generated by superglue fuming should only be executed by trained laboratory technicians. The laboratory should be informed as to what chemical treatments were applied to the submitted items as well as the duration of the fuming process. I recommend communicating with the laboratory before submitting items for processing.

Caution must be taken to use sterile forensic tools that are clear of DNA. Processing tools consist of 1) reusable magnetic wands with disposable plastic applicators, 2) DNA-clean containers of magnetic and standard fingerprint powders, to include fluorescing powders, 3) nitrile gloves, 4) protective coveralls, such as, MicroMax, 5) fluid-resistant surgical masks (i.e. 3M N95 1860), 6) hair nets (hooded coveralls can provide the same shielding), 7) face shields (for those with beards), and 8) footwear covers. This basic list of material should be supplied to all crime scene technicians. It is relatively inexpensive as seen by some of the examples listed below.

The items listed below, in my opinion, will provide the user with the best results in the collection and recovery of DNA in the field:

• Reusable magnetic wands and disposable plastic applicators can be purchased in packages. For example, Doje's Forensic Supplies3 TEX Magnetic Applicator disposable covers (5-pack) for $3.61. As seen in the image below, these are disposable magnetic applicators, fiber and feather dusters, and applicators.

• Protective coveralls, such as Lynn Peavey Company’s MicroMax Protective Coveralls4, are generally available with a cost of approximately $6.25 a unit.

Editor’s Note: Due to the high demand for personal protective equipment (PPE) in the coronavirus pandemic, pricing and availability is unpredictable.

Personal protective equipment (PPE) as seen in FBI ERT Flight 93 crash site.

Coveralls and personal protective breathing appliances are a must, not only for the safety of the personnel processing the scene but also to provide a DNA proactive contamination barrier. These can be the most-costly items. The purchase of these items should be purchased in bulk through any of the bigger forensic supply companies (just putting the word “forensic” on an item seems to double the cost). An alternative that may lower the price would be to purchase through a medical-supply company. It is the responsibility of the department to provide PPE to officers and technical agents in the field under OSHA 29 CFR Part 1910.

From 1991 through to the 2015, as a practitioner and police instructor, I worked under the assumption that cyanoacrylate did destroy DNA. Law enforcement in general was restrained by the fear that recovering a latent fingerprint meant they couldn’t recover DNA through swabbing, and vice versa. Extreme caution was taken on most items that were collected at the scene. Recovery of weapons is a good example.

As an FBI Special Agent specializing in evidence recovery from 1991 into 2003, I had to choose between latent impressions or DNA. In most violent crime scenes, bloodstain, trace evidence, and fingerprint evidence are present. The choice between latent fingerprints and DNA collection at the scene has been simplified as a result of the laboratory being able to recover DNA after the superglue fuming process.

To maintain scene integrity, before items are recovered and collected by crime scene technicians and law enforcement, agents need to make sure that items chosen to be dusted, fumed for latent fingerprint impressions, and/or swabbed for DNA are photographed in detail (orientation, midrange, and close-up). Skipping any one of the photo steps may be a point of entry in court for the defense in cross examination to lay a foundation of contamination issues as well as failure of the officer to follow standard processing procedures. The contamination or addition of DNA to an area to be swabbed can lead to an aggressive cross examination which can weaken the prosecution's case by causing doubt in the minds of the jurors. The addition of genetic material, as a result of unintentional transferences, is probably the most significant factor confronting law enforcement officers in the processing of scenes.

A case in point: State of Wisconsin vs. Miguel A.F. Navarro (Pierce Co. Case No. 2018CF000156) dealt with the August 6, 2018 murder of Israel Valles-Flores at the hands of Miguel Navarro. Navarro's weapon was a circular saw. Navarro and Flores were re-roofing a home. The scene of the crime started at the peak of the roof on the second story and descended to an attached garage roof on the first story. Local police and county sheriff deputies responded to the scene. The sheriff’s department took over control of the scene, processing, and overall investigation. On December 5, 2019, Navarro pleaded guilty to one count of reckless homicide in Pierce County, Circuit Court.5

In police jargon, the case was a “slam dunk”. The homicide occurred in broad daylight in front of two witnesses, as seen in the crime scene photographs below.

This image shows the north main roof of the residence after the assault. The red oval is the general area where the assault started.

The victim was found on the roof at the northwest corner of the south garage roof.

The physical evidence was overwhelming, but it could have turned disastrous if any number of evidentiary exhibits had been found to be contaminated or if the collection was improperly done—especially in the area of fingerprints and DNA.

An area where defense attorneys could have possibly attempted to sway the jury are seen in crime scene photographs, starting with the image below. In this photograph, we observe sheriff detectives photographing and processing the scene in their summer uniforms and mismatched footwear.

If the officers are not wearing protective booties (covers) within a scene, the possibility of generating misleading footwear impressions increases. If officers are not going to wear shoe covers, they need to be wearing the same footwear, so that if footwear impressions are recovered, they can easily be compared and eliminated. If SWAT and first responders have been equipped with the same brand and style of shoe, it becomes much easier to eliminate and compare recovered footwear transfer impressions.

Protective gowning (or PPE) should be mandatory in all violent scenes, not only to prevent officers from contaminating the scene but, more importantly, to decrease the chance of biohazardous material and trace evidence from being transferred to them, their patrol vehicles, the department, and the officers’ homes.6 The department has a “responsibility for developing programs to protect workers.” Based upon reviews of the pertinent data, OSHA 29 CFR Part 1910 states that “certain employees face a significant health risk as the result of occupational exposure to blood and other potentially infectious materials because they may contain bloodborne pathogens.”7

Personal safety and contamination of evidence are the primary issues when officers work in an extremely bloody scene without wearing protective clothing, face shields, eye protection, or shoe covers. OSHA standards come in to play within violent crime-scene environments—especially when dealing with bloodborne pathogens.8 In such a case, the department could be placing officers in an unsafe position where they could be contaminated and become instruments of contamination (by bloodborne bio-pathogens associated with the victim whose medical history is unknown). The department could open itself to civil suit and workers’ compensation claims. The officers working such a scene who have not been provided with personal safety equipment become carriers and may become infected. This leaves their department vulnerable to civil suit.

The officer processing the rooftop murder scene wore the same evidence gloves throughout the processing of this incident. The likelihood of cross-contamination increased the longer they were in this scene without the proper safety barriers to shield them and the evidence they were attempting to recover. The more they processed the scene with only the one protective glove layer, the more items within the scene were cross-contaminated. The officers could have unintentionally transferred trace and biological evidence by collecting and packaging evidence while wearing the same gloves throughout the processing of the scene.

The outside of the packaged item(s) now become a carrier of bio and trace evidence. The item(s) can be compared to a ticking time bomb. Contaminates are invisible, and their very nature is transitory. Each person handling and transporting the contaminated item becomes a potential carrier (indirect transfer) of trace and bio evidence. This is especially true of the evidence custodian who will, in his normal course of business, be in direct contact with the packaged evidence.

The image below is taken from several crime scene photographs.

There are three potential modes of contamination:

  1. Incorrect collection techniques such as incorrect packaging of evidence. Examples include clothing that lacks individual packaging, allowing indirect transfer of trace and biological evidence. Another example is placing multiple wet exhibits in drying cabinets. This, in turn, generates a situation of cross contamination.
  2. Biological and trace evidence transfer by officers not wearing face shields, beard covers, hair nets, shoe covers, and micro-protective coveralls can cause DNA cross-contamination.
  3. When processing, consider environmental factors pertaining to weather and a hazardous work environment (i.e. building construction site).

About The Author
John Louis Larsen served as a Special Agent with the FBI for 22 years and was one the founders of the FBI's Evidence Response Team (ERT) program. His last duty assignment was with the FBI'S Chicago Division as Senior ERT Leader. Larsen currently is President of Larsen Forensics & Associates. He has worked as an Investigator for the Special Prosecutors Office of Cook County and was a Senior Forensics Consultant with Quest Consultants International, Ltd.; a sworn officer with the Office of the Special Prosecutor of Cook Count (Illinois); and a training instructor for Sirchie Laboratories in use of the Reflective Ultraviolet Imaging System (RUVIS). He is also an adjunct instructor for the Homeland Security Training Institute at the Suburban Law Enforcement Academy / College of DuPage. He is also a contributing author to Sanford Weiss' forthcoming book, Forensic Photography for the Preservation of Evidence from CRC Press. He can be reached at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it and at 630-418-0031.


1. Karl Reich, Ph.D., Chief Scientific Officer, Independent Forensics of Illinois, 500 Waters Edge, Suite 210, Lombard, Illinois 60148.

2. The Science of Fingerprints, FBI, United States Department of Justice, Revised 11-79), Chapter 13, Latent Impressions, page 175.

3. www.Dojes.com

4. www.LynnPeavey.com

5. Article by Chris Gray entitled Roofer Pleads Guilty in Wisconsin Rooftop Homicide Case, December 10, 2019, www.roofingcontractor.com/articles/94092-roofer.

6. Safety for the Forensic Identification Specialist 2nd Edition by Nancy E. Masters, Master Consultations, Lightning Powder Company, Inc., 13386 International Parkway, Jacksonville, Florida 32218, Chapter 5, Biological Hazards, Section – Universal Hazards, pages 43-44.

7. Ibid, Chapter 5, Biological Hazards, Section – Universal Hazards, pages 43-44.

8. Ibid, Employer Responsibilities, pg. 45.

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