DNA evidence in modern criminal investigations is ubiquitous: You would be hard-pressed to find a case in which some sort of biological evidence does not come into play. Along with the usual lineup of blood, semen and saliva, “touch DNA” samples now make up more and more of the DNA analyst’s case load. As a law enforcement officer, what should you know about DNA evidence?
The following information is meant for any officer who comes into contact with a crime scene. It can be used as a guide to help ensure potential DNA-bearing evidence is recognized as such and that it’s collected, packaged, stored and transported in a manner that minimizes contamination, loss and damage of the evidence.
Recognizing Possible DNA Sources
Not all biological evidence is visible to the naked eye. In fact, the majority is not. Granted, a bright red blood stain can be pretty obvious, but even blood can be small enough and dilute enough that it’s not visible.
Click here to view examples of potential DNA-bearing evidence that may be found at various crime scenes. It’s important that evidence that might contain “invisible” biological fluid be collected and sent to the lab for analysis.
Collecting Evidence to Minimize Contamination
When collecting evidence at the crime scene, the officer must take steps to minimize the possibility of contamination. Contamination is defined as the accidental transfer of DNA. This transfer could be DNA from one item of evidence to another, or it could be DNA from the officer to the evidence.
Tips to help avoid contamination while collecting evidence include:
- Always wear disposable gloves when collecting evidence, and change them often. They should be changed:
- Between each item collected.
- After using your cell phone or radio.
- After touching your face, eyes, nose or hair.
- If you use your gloved hand to cover a cough, sneeze or yawn.
2. Handle the evidence as little as possible. This helps prevent contamination and helps prevent the loss of touch DNA evidence.
3. Use clean or disposable tools (e.g., scissor, forceps, swabs) to collect the evidence.
4. Don’t talk, cough or sneeze over the evidence.
5. Take great care in collecting evidence if you have dandruff or a peeling sunburn because your DNA may fall onto the evidence.
Packaging Evidence Properly
The goal here is to prevent loss or damage to the evidence. Some tips for proper packaging:
- Never use plastic. Biological evidence must be allowed to breathe. Airflow is important; therefore, choose paper bags, manila envelopes or cardboard boxes.
- Swabs can be taken from large immovable objects, but it’s often best to submit the item as a whole to the lab for examination.
- Clothing should be submitted as is. Don’t cut out any possible blood stains because blood-spatter analysis may be important to the case.
- Package each item separately, such as all the clothing items from the victim or suspect, even if all the items are from one person. Each item should be placed in a separate bag to avoid transfer of biological fluid from one item to the next.
- Hairs should be packaged in a paper fold inside an envelope. If they are placed in an envelope by themselves, they may fall out during transport.
- Guns and knives should be zip-tied into specially made cardboard gun boxes. Before packaging the gun, render the weapon safe.
Storing & Transporting Evidence
Biological evidence is sensitive to such environmental factors as sunlight, heat, humidity, moisture, burning or mixture with soils. The less time the evidence spends exposed to the elements, the more likely a DNA profile will be obtained.
- Properly seal the evidence or it may not be accepted by the lab.
- Don’t leave evidence in the trunk of a hot patrol car! Get the evidence to the lab or to your evidence storage area ASAP.
- Most biological evidence should be stored refrigerated or frozen to prevent degradation and deterioration.
Finally …
A closing suggestion: communicate. Communicate regularly with the DNA analyst assigned to your case. Provide them with a scenario of the crime so they can be a resource to you in determining what might be the most probative items of evidence to test.
