Scent identification in crime scene investigation
Scent identification in crime scene investigation involves the use of specially trained dogs to detect unique odors associated with individuals. This technique leverages the canine sense of smell, which can be significantly more sensitive than that of humans, allowing dogs to track or identify suspects based on scent. Dogs can follow the scent trail of an individual, using indicators such as disturbed ground or crushed vegetation, and can identify a person by smelling a scent sample linked to them. Research indicates that humans possess stable and unique odor profiles, influenced by genetic factors, nutrition, hygiene, and health.
One of the innovative applications of this ability is the scent identification lineup, which is gaining traction in law enforcement, particularly in Europe. This procedure allows dogs to match scents from crime scenes to individuals, although results can vary based on the age of the scent sample. While scent identification can establish connections between suspects and crime scene evidence, it is limited in that it does not prove participation in a crime. In the U.S., scent evidence is typically viewed as a piece of reasonable suspicion rather than definitive proof, with varying opinions on its admissibility in court. Overall, scent identification serves as a valuable tool in criminal investigations, yet it is most effective when corroborated by additional evidence.
Scent identification in crime scene investigation
DEFINITION: Use of specially trained dogs to identify particular persons by their unique odors.
SIGNIFICANCE: In some cases, criminal investigators use trained scent identification dogs to connect crime scene evidence with suspects.
The canine sense of smell can be as much as ten thousand times more sensitive than the human sense of smell, and dogs’ ability to discriminate among odors is sometimes useful during criminal investigations. Traditionally, have used canines’ olfactory skills to track or trail criminal suspects as well as missing persons. Dogs can track a person by following indicators of a fresh track, such as the odors of crushed vegetation and disturbed ground. To trail an individual, a dog needs a scent sample of the quarry.
A trailing dog uses the scent of a known person to identify that person’s location. If presented with the scent of an unknown person, a trained dog can identify that person in a crowd. This is the basis of scent identification.
Human Scent
Research has shown that humans have stable and unique odor profiles and that dogs can be trained to recognize unique scents in mixtures of other odors. Studies with bloodhounds indicate that genetics plays a key role in determining the uniqueness of a person’s scent. Although dogs can discriminate among members of human families, identical twins, who essentially have the same genes, pose a problem for dogs. It has been suggested that a human’s scent profile, or “odortype,” may be determined by the expression of genes in the major histocompatibility complex, the genes that also control immune responses through recognition of “self” and “nonself.”
Other elements that may contribute to the uniqueness of an individual’s scent include nutrition, hygiene, and state of health. Such secondary factors may become significant if a dog must distinguish between identical twins.
Studies with Dutch and German scent identification dogs have found that the age of a scent sample affects a dog’s performance. Dogs faultlessly matched odors collected on the same day of the testing, but they made mistakes when presented with scent samples that had been stored for two weeks. After this initial drop in performance level, the dogs’ accuracy remained steady even with scent evidence up to six months old. These results show that scent has a volatile component.
Using gas chromatography-mass spectrometry, scientists have analyzed the volatile components of human odor signatures. They have found that the distinctiveness of humans’ scents arises from a combination of common compounds that differ in amounts from person to person. In addition, they have discovered that some compounds are unique to certain people.
The Federal Bureau of Investigation (FBI) has supported bloodhound research workshops in which the durability of human scent has been examined. One study showed that a dog could use the scent of a mailed letter to locate the letter writer’s house six months after the person had moved out of the house. Another study found that bloodhounds could detect human scent on bomb fragments and identify the individual who had handled the bomb before it exploded.
The Scent Identification Lineup
The durability of human scent can combine with the sensitivity of the canine nose to allow criminal investigators to link a and crime scene evidence in a scent identification lineup. This technique is relatively new to law-enforcement agencies in the United States, but it has become established in Europe. The Netherlands National Police Agency, for example, has formulated a protocol for the scent that is accepted by the Dutch courts.
The basic Dutch police scent lineup proceeds as follows. A scent identification dog and its handler enter a test room that contains six stainless-steel scent-carrier tubes, each 10 centimeters (about 4 inches) long, clamped to platforms; one of these tubes has been handled by a suspect, and the other five have been handled by adults who are not associated with the suspect. After several quality-assurance tests are conducted, the suspect identification stage begins with the dog sniffing a scent evidence object collected from the scene of the crime. The dog then smells the scent-carrier tubes. If the dog picks the scent-carrier tube handled by the suspect in two tests, the police conclude that the scent evidence object and the suspect share an “odor similarity.”
The scent lineup protocol requires that the dog’s handler be unaware of who touched the scent-carrier tubes. This protects the findings from being invalidated by the possibility that the handler gave any nonverbal, or even unconscious, suggestion to the dog.
Dogs remain the best tools for odor identification. However, scientists have developed a test for use in the laboratory that can identify human odor profiles as belonging to men or women.
Scent Identification Evidence in US Courts
The identification of a person by scent has an important limitation: Scent can be transferred from one person or object to another. Although scent identification can establish a direct or indirect link between an individual and crime scene evidence, it cannot prove that the individual participated in a crime.
In the United States, scent identification is usually not considered to be enough evidence to justify an arrest. Rather, a scent identification is viewed as one indicator of reasonable suspicion. Investigators must seek corroboration of a scent identification to meet the standard of before they can the identified person.
Judges in the United States have expressed different opinions about the admissibility of scent lineup evidence. In some jurisdictions, judges view scent identification as sufficiently reliable to be presented during trial, whereas judges in other jurisdictions require the presentation of scientific evidence that every person has a scent so unique that it provides an accurate basis for identification. Although judges disagree about the scientific proof of reliability, scent identification offers a valuable tool for criminal investigation.
Bibliography
Harvey, Lisa M., et al. “The Use of Bloodhounds in Determining the Impact of Genetics and the Environment on the Expression of Human Odortype.” Journal of forensic Sciences 51, no. 5 (2006): 1109-1114.
Harvey, Lisa M., and Jeffrey W. Harvey. “Reliability of Bloodhounds in Criminal Investigations.” Journal of Forensic Sciences 48, no. 4 (2003): 811-816.
Schoon, Adee, and Ruud Haak. K9 Suspect Discrimination. Calgary, Alta.: Detselig Enterprises, 2002.
Syrotuck, William G. Scent and the Scenting Dog. 4th ed. Mechanicsburg, Pa.: Barkleigh Productions, 2000.
Tomaszewski, Tadeusz, and Piotr Girdwoyn. “Scent Identification Evidence in Jurisdiction.” Forensic Science International 162, nos. 1-3 (2006): 191-195.
Willis, Claudia Suzanne. "Odor Profiles and the Future of Forensic Evidence." Science Connected Magazine, 8 Aug. 2023, magazine.scienceconnected.org/2023/08/odor-profiles-and-the-future-of-forensic-evidence/. Accessed 19 Aug. 2024.