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The Benefits of Scent Evidence.

In July 1998, a migrant worker in Northern California lured two men, a woman, and the woman's son and daughter into a cow pasture, where he had prepared a grave site 2 weeks earlier. First, the suspect shot the men and placed them into the grave. Then, he restrained the woman and sexually assaulted her in front of her two small children. While the suspect bludgeoned the boy and buried him, the woman managed to escape. After burying the young girl alive, the suspect fled.

Fortunately, within 24 hours of this quadruple murder, scent-discriminating dogs found the material used to restrain the woman, located the grave site, and provided investigators with leads indicating the suspect's whereabouts. By following the suspect's scent trail and using eyewitnesses accounts, investigators identified a pay telephone the suspect had used. From information obtained from subpoenaed telephone records, they apprehended the suspect in Southern California.

Though often overlooked at a crime scene, scent evidence can help investigators solve a number of cases. With an understanding of its capabilities, law enforcement officers can use scent evidence as an investigative tool.

Understanding Scent

By definition, scent is the "bacterial, cellular, and vaporous debris enshrouding the individual."(1) Know collectively as "raft," this debris consists primarily of dead or dying skin cells, which the body sheds at a rate of approximately 40,000 each minute.(2) Air currents project the raft upward and away from the body, much like a plume. The debris becomes deposited in the environment in a conically shaped pattern known as the scent trail.

On average, humans lose approximately 1 to 2 1/2 quarts of water through perspiration each day, with that amount greatly increasing as activity and temperature intensify.(3) Becoming saturated with perspiration and skin oils, raft serves as a "breeding ground" for the bacteria normally found on the skin.(4) This action of bacteria on cellular debris may, in fact, form the basis of the human scent trail. Although culture, diet, environment, heredity, and race influence it to some degree, the combination of bacteria, vapor, and cellular debris is believed to be unique to the individual, accounting for the singularity of human scent.

Using Scent Evidence

The uniqueness of human scent permits law enforcement to use scent evidence to

1) follow a suspect directly from a crime scene;

2) determine a suspect's direction of travel from a crime scene, possibly locating additional evidence that the suspect drops, abandons, or unwittingly leaves behind (e.g., footprints or tire tracks);

3) ascertain the whereabouts of a suspect;

4) identify a suspect in a lineup;

5) place a suspect at a particular crime scene or location;

6) establish probable cause; and

7) locate and recover missing persons, whether dead or alive.

Law enforcement agencies cannot rely on scent evidence alone to accomplish these tasks; they must use specially trained dogs to link the evidence to the individual and the crime.

Working with Scent-Discriminating Dogs

Dogs have an extraordinary number of olfactory sensory cells (220 million in a sheep dog, compared to 6-10 million in humans), enabling them to smell 44-100 times better than human beings/Although law enforcement agencies have used a number of breeds in police work, the bloodhound, with its keen sense of smell and innate determination, remains the best suited for locating individuals and evidence.(6)

A properly trained dog can successfully follow trails of up to 10 days old. Still, because human scent is affected by such environmental elements as wind, temperature, humidity, and other factors and also diminishes with time, a dog should begin working within 24 hours. Law enforcement agencies that bring dogs into an investigation right away increase the odds of preserving vital evidence.

Preserving the Crime Scene

Law enforcement first responders should make every, effort to preserve the crime scene or the last known location of a missing person ("point last seen"). To do this, they first should limit access to the area. A perimeter of at least 200 feet around the scene can prevent contamination and give dogs and their handlers an adequate area in which to obtain a scent trail. Because automotive exhaust masks scent, individuals arriving at the scene should turn off their vehicles' engines.

Pets also should be confined. Although keeping individuals from contaminating the crime scene may prove particularly difficult in some cases - for example, in missing-child investigations, where anxious family members and friends gather at the scene - doing so gives law enforcement the best chance to collect critical evidence.

Collecting Scent Evidence

Anything a suspect has touched, worn, or eliminated (e.g., bodily fluids, including blood and urine) can serve as scent evidence, but articles of clothing worn close to the skin work best. Investigators should avoid collecting clothing from hampers because it likely contains the scent of other family members or roommates.

Investigators or evidence technicians should collect the scent evidence, handling it with clean, unpowdered latex gloves and placing it in clean plastic (e.g., food storage) or paper bags. Garbage bags often contain odor-inhibiting coatings, which make them unacceptable evidence storage devices. The bags should be properly sealed, labeled, and stored for future use, preferably in a refrigerator or freezer. Agencies also may use a plastic bag made specifically for collecting evidence that can be heat sealed.

One technique used to minimize contamination involves taking a clean plastic bag, placing a hand on the bottom of the bag, pushing the bottom through the bag, and grasping the scent material. The investigator or evidence technician then pulls the material back into the bag and seals it.(7)

Investigators can use scent pads to obtain evidence from items they cannot take from the scene. They simply take a sterile gauze sponge and place it on or wipe it across an article with which they believe the suspect has had contact. The longer the sponge sits on the item, the better. The same evidence-gathering rules used for other scent evidence apply to the retrieval and storage of scent pads.

An officer with the Niagra County, New York, Sheriff's Office has designed a device that vacuums scent from an object onto a sterile gauze pad, leaving fingerprints and other trace evidence intact.(8) In one case, a man had been suspected of the kidnapping and attempted murder of his former girlfriend's new suitor. Personnel from the sheriff's office used the machine to obtain scent evidence from the vehicle from which the suspect allegedly had abducted the victim. The evidence was then stored and later used to identify the suspect when he arrived at the sheriff's office for a consensual interview. A bloodhound, once scented, identified the vehicle that the suspect had driven to the interview and successfully trailed him into the interview room.

Presenting Evidence in Court

Because state laws vary, law enforcement officers should consult their department's legal advisors or local prosecutors before using scent evidence. In general, however, the courts have accepted scent evidence, provided that agencies meet certain conditions. For example, at least one court has ruled that an agency must establish the qualifications of the handler and the dog and properly protect the crime scene.(9) Another court ruled that scent evidence alone cannot support a conviction.(10) In another case, the court ruled that evidence of a dog's tracking is admissible provided that the dog has been trained to track humans; it has proven reliable in such training; and the handler places the dog on the trail within the dog's period of efficiency and in a location where evidence or circumstances indicate the suspect has been.(11) In all of these cases, a common denominator exists: dog handlers must document their own and their dogs' experience, skill, training, and reputation when called upon to do so.


The scene of a crime often contains sufficient evidence to convict the offender. Yet, investigators trained to look for visual clues may miss other important evidence, namely scent. Because the scent of every human being is nearly as unique as a fingerprint, scent evidence can help investigators find missing persons, uncover evidence, and locate suspects and link them to specific crimes.

Properly recognizing, collecting, preserving, and presenting scent evidence remain key to its use as an investigative tool. An experienced handler and well-trained dog increase the odds for the successful discovery and application of scent evidence.

Law Enforcement Scent Evidence Resources

These associations represent a sample of the organizations that provide assistance and training in scent evidence to law enforcement officers, administrators, and handlers. These organizations specialize in the use of bloodhounds.

National Police Bloodhound Association

Cpl. Doug Lowry

Maryland State Police


Law Enforcement Bloodhound Association

Brian Joyner

Veteran's Administration Police

Tomah, Wisconsin, 608-372-1706


1 William G. Syrotuck, Scent and the Scenting Dog (Rome, NY: Arner Publications, Inc., 1972), XIII.

2 Ibid., 37.

3 Patricia Deuster, Anita Singh, and Pierre A. Pelletier, The Navy SEAL Nutrition Guide (Bethesda, MD: Uniformed Services University of the Health Sciences. 1994), 51.

4 Milo D. Pearsall and Hugo Verbruggen, M.D., Scent: Training to Track. Search, and Rescue (Loveland, CO: Alpine Productions, 1982), 15.

5 Piet Vroon, with Anton von Amerongen, and Hans de Vries, Smell: The Secret Seducer, trans. Paul Vincent (New York: Farrar, Straus, and Giroux, 1994), 27.

6 The National Police Bloodhound Association provides search and rescue assistance and conducts training seminars for law enforcement bloodhound handlers. For more information, contact Cpl. Douglas Lowry, Maryland State Police, 301-733-0951.

7 Bill Tolhurst, The Police Textbook for Dog Handlers (Sandborn, NY: Sharp Printing, 1991), 33.

8 For more information, contact Bill Tolhurst, 383 Willow Street, Lockport, NY 14094-5512, 716-434-4126; for orders, contact Larry Harris, 1807 Highland Drive, Newport Beach, CA 926604402, 949-548-0782.

9 Terrell v. Maryland, 239 A.2d 128 (Md. App. 1968).

10 People v. Perryman, 280 N.W.2d 579 (Mich. App. 1979).

11 People v. Centolella, 305 N.Y.S.2d 460 (Oneida County Ct. 1969).

Formerly assigned to the FBI's Critical Incident Response Group as coordinator of the K-9 Assistance Program, Special Agent Robert Hunt currently serves in the Practical Applications Unit at the FBI Academy. For information on the FBI's K-9 Assistance Program, contact Special Agent Gregory Cox at 703-632-1539.
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Article Details
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Author:Hunt, Robert
Publication:The FBI Law Enforcement Bulletin
Geographic Code:1USA
Date:Nov 1, 1999
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