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Crime Scene and DNA Basics for Forensic Analysts

Location & Collection of Evidence

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Items of physical evidence are not always visible to the naked eye and may be easily overlooked. A deliberate, methodical, disciplined approach to collection and preservation of evidence is essential. One exception may be if evidence integrity is at risk, and under those circumstances it is important that rapid decisions be made to prevent its degradation and/or loss.

It is imperative that the investigator obtain as much information as possible regarding the circumstances of the crime prior to entering the scene. Statements from witnesses, victims, or first responders can provide a broader understanding of the investigation. The investigator can develop an approach to the scene based on this information and the nature of the crime. For example, at the scene of a burglary, attention may focus on the point of entry. Fragments of wood, metal, or broken glass may be discovered, along with fingerprints, blood, and fibers from clothing deposited when the perpetrator forced entry.

In the case of a violent crime such as a sexual assault, attention may be directed to the clothing and the person of the victim(s) and the suspect(s). An investigator might find body fluids, stains, torn clothing, fingerprints, fibers, hair, and other trace materials in the areas where the attack took place. Potential evidence such as saliva, bite marks, semen, hair, skin tissue under the finger nails, and other trace materials may be found on the victim(s). Transferred evidence such as cosmetics, vaginal fluid, hair from the victim, and blood may also be found on the suspect.

Once potential evidence is located and documented, the next step is to collect and package the items in a manner that prevents contamination, loss, and deleterious change.

Biological evidence requires care to guard against the possibility of cross contamination either by the investigator or by other biological specimens at the scene. Equipment is available to crime scene investigators which aide in the prevention of cross contamination.

Types of equipment include:

  • Tyvek white paper body suit
  • Paper mask that covers nose and mouth
  • Eye protection
  • Latex or Nitrile gloves
  • Sleeve protectors
  • Shoe covers
  • Hair net

The investigator should prioritize the order in which evidence is collected. Biological evidence, trace materials, and evidence of a fragile nature should be collected first. Collection methods used to gather and package this evidence vary. The use of an alternate light source (ALS) or oblique lighting may be necessary. A sample detected with the ALS should be properly packaged with a notation alerting the analyst that it is a luminescent sample.

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