Contaminated evidence

Contamination is the introduction of something to a scene that was not previously there.[1] This means trace materials are added to a crime scene after the crime is committed. This can happen before, during and after authorities take samples of the evidence from a scene. Many people can contaminate the evidence at a crime scene, including witnesses, suspects, victims, emergency response personnel, fire fighters, police officers and crime scene investigators. Juries expect to see forensic evidence before they make a decision in a case which relies on that evidence. Because of this, attorneys on both sides try to discredit forensic evidence that does not support their clients' interests. This requires crime scene investigators be especially careful to guard against contamination in the forensic samples taken from a crime scene. A miscarriage of justice can occur when these procedures are not carried out carefully and accurately.[2]

Avoiding contamination

Evaluation of the crime scene

Evaluating a scene before anyone enters can be key to keeping contamination to a minimum.[1] The examination of the scene will usually begin with a walk through of the area along the "trail" of the crime. The trail is that area which all apparent actions associated with the crime took place. The trail is usually marked by the presence of physical evidence. This may include the point of entry, the location of the crime, areas where a suspect may have cleaned up, and the point of exit. In some cases, a walk through may become secondary if potential evidence is in danger of being destroyed. In that case, this evidence should be preserved, or documented and collected as quickly as possible.[3]

The purpose of the walk through is to note the location of potential evidence and to mentally outline how the scene will be examined. The walk through begins as close to the point of entry as possible. The first place the investigators should examine is the ground on which they are about to tread. If any evidence is observed, then a marker should be placed at the location as a warning to others not to step on the item of interest. As the walk through progresses, the investigators should make sure their hands are occupied by either carrying notebooks, flashlights, pens, etc. or by keeping them in their pockets.[3] This is to prevent depositing of unwanted fingerprints at the scene. As a final note on the walk through, the investigators should examine whatever is over their heads (ceiling, tree branches, etc.). These areas may yield such valuable evidence as blood spatters and bullet holes. Once the walk through is completed, the scene should be documented with videotape, photographs, and/or sketches.[3]

Protecting the crime scene

The most important aspect of evidence collection and preservation is protecting the crime scene. This is to keep the pertinent evidence uncontaminated until it can be recorded and collected. Eating, drinking, or smoking should never be allowed at a crime scene.[4] A command post should be set up for such purposes. The post is to be set up somewhere outside the restricted areas. It could be a vehicle, picnic table, hotel room, tent, etc. It can be used as a gathering place for non-involved personnel, a place for investigators to take breaks, eat, drink, or smoke, a communication center, a place for press conferences, a central intelligence area, etc.[4] Protection of the crime scene also includes protection of the crime scene investigators. One person, whether a civilian or a police crime scene investigator, should never be left alone while processing the scene. This is especially true if the suspect has not been apprehended. There are many stories of suspects still hiding at or near their area of misdeed. That is why there should always be at least two people working the scene. At least one of these people should have a radio and a firearm.[4]

Documentation of the crime scene

If available, a video camera is the first step to documenting a crime scene. Videotape can provide a perspective on the crime scene layout which cannot be as easily perceived in photographs and sketches. It is a more natural viewing medium to which people can readily relate, especially in demonstrating the structure of the crime scene and how the evidence relates to the crime. The video camera should have a fully charged battery as well as date and time videotape display functions. The taping should begin with a general overview of the scene and surrounding area. The taping should continue throughout the crime scene using wide angle, close up, and even macro (extreme close up) shots to demonstrate the layout of the evidence and its relevance to the crime scene.[3]

Photographs can demonstrate the same type of things that the videotape does, but photographs from the crime scene can also be used in direct comparison situations. For example, actual size photographs (also known as one-to-one photos) can be used to compare fingerprint and shoeprints photographed at the crime scene to known fingerprints or shoes from a suspect. This is the advantage of photographs over videotape.[3]

The final phase in documenting the scene is making a crime scene sketch. The drawback of photographs is that they are two-dimensional representations of three-dimensional objects. As a result, most photographs can distort the spatial relationships of the photographed objects causing items to appear closer together or farther apart than they actually are. If spatial relationships of the evidence are important or if something needs to have proportional measurements included in it for calculations (such as bullet trajectory angles, accident reconstructions, etc.) then a sketch must be made of the crime scene.[3]

Collection of evidence

Once the crime scene has been thoroughly documented and the locations of the evidence noted, then the collection process can begin. Most items of evidence will be collected in paper containers such as packets, envelopes, and bags.[5] Liquid items can be transported in non-breakable, leakproof containers. Arson evidence is usually collected in air-tight, clean metal cans. Only large quantities of dry powder should be collected and stored in plastic bags. Moist or wet evidence (blood, plants, etc.) from a crime scene can be collected in plastic containers at the scene and transported back to an evidence receiving area if the storage time in plastic is two hours or less and this is done to prevent contamination of other evidence. Once in a secure location, wet evidence, whether packaged in plastic or paper, must be removed and allowed to completely air dry. That evidence can then be repackaged in a new, dry paper container. Under no circumstances should evidence containing moisture be packaged in plastic or paper containers for more than two hours. Moisture allows the growth of microorganisms which can destroy or alter evidence. Any items which may cross contaminate each other must be packaged separately. The containers should be closed and secured to prevent the mixture of evidence during transportation. Each container should have: the collecting person's initials; the date and time it was collected; a complete description of the evidence and where it was found; and the investigating agency's name and their file number.[5]

The following is a breakdown of the types of evidence encountered and how the evidence should be handled: Fingerprints (also includes palm prints and bare footprints) are the best evidence to place an individual at the scene of a crime. Collecting fingerprints at a crime scene requires very few materials, making it ideal from a cost standpoint. All non-movable items at a crime scene should be processed at the scene using gray powder, black powder, or black magnetic powder. Polaroid 665 black and white film loaded in a Polaroid CU-5 camera with detachable flash should be used to make one-to-one photographs of prints which do not readily lift. All small transportable items should be packaged in paper bags or envelopes and sent to the crime lab for processing. Because of the "package it up and send it to the lab" mentality, some investigators skim over collecting prints at a crime scene. Collecting prints at the crime scene should be every investigator's top priority. Fingerprints from the suspect as well as elimination fingerprints from the victim will also be needed for comparison (the same holds true for palm and bare footprints).[5] Bite marks are found many times in sexual assaults and can be matched back to the individual who did the biting. They should be photographed using an ABFO No. 2 Scale with normal lighting conditions, side lighting, UV light, and alternate light sources. Color slide and print film as well as black and white film should be used. The more photographs under a variety of conditions, the better. Older bitemarks which are no longer visible on the skin may sometimes be visualized and photographed using UV light and alternate light sources. If the bitemark has left an impression then maybe a cast can be made of it. Casts and photographs of the suspect's teeth and maybe the victim's teeth will be needed for comparison. For more information consult a forensic odontologist. Much like a bullet that has individualizing striations on it, natural fingernails have individualizing striations on them.[5] A broken fingernail found at a crime scene can be matched to the individual it came from many months after the crime has been committed. Broken fingernails should be placed in a paper packet which is then placed in a paper envelope. It can then be transported to the crime lab for analysis. Known samples from the suspect and maybe from the victim will be needed for comparison.[5] Handwriting samples can also be matched back to the individual that produced them. Known exemplars of the suspected person's handwriting must be submitted for comparison to the unknown samples. Questioned documents can also be processed for fingerprints. All items should be collected in paper containers. For more information consult a questioned documents examiner.[5] Fracture matches can positively link broken pieces at the scene with pieces found in the possession of a suspect. For example, headlight fragments found at the scene of a hit and run could be positively matched to a broken headlight (just like putting together a jigsaw puzzle) on a suspect's vehicle. Larger fragments should be placed in paper bags or envelopes. Smaller fragments should be placed in a paper packet and then placed in an envelope.[5] If a root sheath is attached, then DNA analysis using PCR technology can say that this hair came from a certain percentage of the population to which the suspect belongs. If there is no root sheath, then a microscopic analysis can say that the hair has the same characteristics as the suspect's hair and is similar to his or her hair. At this point, no one can say that a hair came from a particular individual. Hair found at the scene should be placed in a paper packet and then placed in an envelope. If a microscopic examination is required, then 15–20 representative hairs from the suspect must be submitted to the lab for comparison. If DNA analysis if going to be used, then a whole blood sample from the suspect must be submitted to the lab in a "Vacutainer." Contact a DNA lab for more information.[5] Fibers can be said that they are the same type and color as those found in a suspect's clothes, residence, vehicle, etc. Fibers should be collected in a paper packet and placed in an envelope. Representative fibers should be collected from a suspect and submitted to the lab for comparison.[5] Paint can be said that it is the same type and color as paint found in the possession of a suspect. Paint fragments should be collected in a paper packet and placed in an envelope. Representative paint chips or samples should be collected from the suspect and submitted to the lab for comparison. Glass can be said that it has the same characteristics as glass found in the possession of a suspect. Smaller glass fragments should be placed in a paper packet and then in an envelope. Larger pieces should be wrapped securely in paper or cardboard and then placed in a padded cardboard box to prevent further breakage. Representative samples from the suspect should be submitted to the lab for comparison.[5]


Not only is the actual crime scene as a whole important to protect in order to avoid contamination the tools used pose just as big of a threat. Simple measures can be taken by an investigator to ensure the integrity of DNA evidence. Tools can easily transfer DNA from one crime scene to another. Fingerprint brushes, for example, can retain the DNA they pick up and create contaminated evidence. In order to ensure there will not be a transfer of DNA on brushes, they should not be reused.[1] Each scene should get a new one. This tip is especially important in situations like homicide cases, where DNA is a crucial factor. Paying the extra six to nine dollars for an extra brush may be the difference in contaminated evidence and alleviating questions. Gloves are another tool that need to be carefully handled in a crime scene. There is no guarantee that gloves will permit contamination. The key is to change gloves often.[1] If one fails to do so, they can be contaminated by obvious reasons like touching blood and other fluids, but also simple movements such as covering your mouth when you sneeze and scratching your face. Like brushes, gloves are extremely cheap and should be disposed of and changed whenever they may have been compromised.


Many trials involve highly technical testimony about the chemistry and biology of evidence and the physics of how it was analyzed.[1] The goal should always be to eliminate questions before they become an issue; aim to never lose a case on a technicality. On cases that go to court, any unexplained evidence collected at the scene needs to be identified. Unidentified evidence such as latent fingerprints, shoe tracks, hair, blood, and DNA swabbing will need to be identified.[1]

Adam Scott

Twenty-year-old Adam Scott was charged with the rape of a woman after his saliva came up on a plastic tray that was reused. Adam was a suspect in a street fight and saliva samples had been taken from him. After a woman was attacked in Manchester a tray with his evidence was reused when in fact it should have been disposed of. This fault in procedure by a worker at LGC caused Adam Scott to be jailed for five months before the mistake was later picked up on.[6]

Time travel murder

A woman, murdered in London in 1997, was taken to the lab for analysis. After searching under her fingernails investigators found a match to another woman whom they suspected to be the killer. However, the woman who came up as a match had been murdered herself three weeks prior to the incident. Investigators were confused as they could not find any correlation between the two women. Finally they came to the conclusion that the mix-up must somehow have stemmed from the forensics lab. After investigating the process in which the DNA was collected they discovered that a single pair of scissors was used to cut both woman's nails. Although they were washed in between, there was still enough DNA to contaminate the scene.[7]

Cory Carroll

Indicted for murder after his involvement in a head-on collision that killed one and injured himself and two others, Cory Carroll was released after evidence was determined not to be admissible. After taking a urine sample at the hospital his results came back positive for high levels of THC. These results were invalid, however, because they were never sent to a state lab to be confirmed. After this there was not enough evidence to charge Cory Carroll of the crime and the case was dismissed.[8]

See also


  1. Warrington, Dick. "Crime Scene Contamination". Forensic Magazine. Retrieved 15 November 2014.
  2. Giannelli, Paul. C. "Wrongful Convictions and Forensic Science". Faculty Publications. Retrieved 18 November 2014.
  3. Schiro, George. "Examination and Documentation of the Crime Scene". Crime Scene Investigator Network. Retrieved 15 November 2014.
  4. Schiro, George. "Protecting the Crime Scene". Crime Scene Investigator Network. Retrieved 15 November 2014.
  5. Schiro, George. "Collection and Preservation of Evidence". Crime Scene Investigator Network. Retrieved 15 November 2014.
  6. Peachey, Paul. "Rape accused Adam Scott was victim of forensics error, regulator finds". The Independent. Retrieved 18 November 2014.
  7. Sullivan, Mike. "The strange case of the 'time travel' murder". BBC News. Retrieved 18 November 2014.
  8. Ritchie, Chris. "Lack of evidence leads to dismissal in murder case". Hazard Herald. Archived from the original on 29 November 2014. Retrieved 18 November 2014.
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