The Importance of DNA in a Sexual Assault Case

Preserving DNA evidence is a key tool for law enforcement’s investigation and prosecution of a sexual assault case. It is used to prove that a sexual assault occurred and to show that the defendant is the source of biological material left on the victim’s body.

Victims should make every effort to save anything that might contain the perpetrator’s DNA, therefore a victim should not:1
  • Bathe or shower
  • Use the restroom
  • Change clothes
  • Comb hair
  • Clean up the crime scene
  • Move anything the offender may have touched

Even if the victim has not yet decided to report the crime, receiving a forensic medical exam and keeping the evidence safe from damage will improve the chances that the police can access and test the stored evidence at a later date.

Learn more about the process of preserving and collecting forensic evidence.

What exactly is DNA?1

DNA is present in all the cells in our body, which determine our individual physical characteristics such as eye, hair, and skin color. DNA evidence is found in material such as blood, saliva, sweat, urine, skin tissue, and semen. DNA could potentially be found on a victim’s body (including under the fingernails), clothing, drinking glass, furniture, weapons, etc. Anything a perpetrator touches could potentially have DNA evidence on it.

DNA evidence is usually collected at a crime scene by trained investigators who make sure the evidence is not damaged. It’s important that law enforcement and investigators receive special training on the handling of DNA evidence to avoid contamination or destruction. DNA evidence can be contaminated, for instance, if it comes into contact with another person’s DNA, or is exposed to heat, humidity, bacteria, and other environmental conditions.

DNA may also be collected from anyone who was known to have been at the crime scene (the victim, responding officer, family member, or witness), as well as anyone the victim had consensual sex with in the 72 hours before the assault.

DNA evidence from the victim’s body and/or clothing is often collected at a hospital or other healthcare facility by a trained professional known as a sexual assault nurse examiner (SANE). If a SANE is not available, another medical professional will collect the evidence. This forensic medical exam (“rape kit exam”) consists of collecting evidence and photographing injuries.

  • Medical attention will also be paid to treating injuries, testing for pregnancy and sexually transmitted diseases. Medications to prevent sexually transmitted diseases and pregnancy and protect against HIV transmission may also be offered.

NOTE: To find a local hospital or healthcare facility that is equipped to collect forensic evidence, contact the National Sexual Assault Hotline (800.656.HOPE). The hotline will connect callers to their local rape crisis center, which can provide information on the nearest medical facility, and in some instances, send an advocate to accompany victims through the evidence collection process.

What happens to the DNA?1

Once the DNA evidence is collected from the crime scene and forensic medical exam, it is either stored away or sent to the crime lab if the victim chooses to pursue a case. The crime lab will use the DNA evidence to develop a DNA profile, which will contain a certain set of identifiers or characteristics specific to this particular DNA strand. Much like fingerprints, DNA profiles are used in criminal investigations to identify individuals who might be involved in a particular crime.

Crime labs and law enforcement officials will compare DNA profiles collected from crime scenes to DNA profiles of potential suspects to see if they match. The main resource of potential suspect DNA profiles is the Federal Bureau of Investigation’s nationwide DNA database system known as CODIS (the Combined DNA Index System)– a system of federal, state, and local databases that contains DNA profiles from known criminal offenders and DNA evidence from past crime scenes. A search will take place for matches between DNA profiles and the DNA found in the current case, hoping to uncover a suspect. While CODIS has a strict privacy policy (protecting information such as names, dates of birth, social security numbers of criminals to be identified), the DNA profile itself is available to examine.

Victims should be aware that even if DNA testing does identify a suspect, it does not solely prove guilt by that criminal. DNA is only part of the investigation; most crimes need other evidence to prove the case. Along those same lines, law enforcement may not be able to locate the suspect to arrest him or her, or the legal time limit for pressing charges (called the “Statute of Limitations”) may have passed.

Each person who handles the evidence (i.e., the SANE, police officer, lab technician) must keep a detailed record of what he or she did with the evidence and the precautions used to prevent its contamination. If you choose to report the crime, this documentation will prove to be very important if the case goes to court. The “Chain of Custody” shows that the evidence was handled carefully and has not been tampered with in any way.

Learn more about receiving medical attention.

  1. DNA & Crime Victims: What Victims Need to Know. The National Center for Victims of Crime. 2008.
This product was supported by grant number 2009-D1-BX-KO23 awarded by the Office for Victims of Crime, Office of Justice Programs, U.S. Department of Justice. The opinions, findings, and conclusions or recommendations expressed in this product are those of the contributors and do not necessarily represent the official position or policies of the U.S. Department of Justice.

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