A call comes in, and officers race to the crime scene. Crime Scene Officers investigating a scene have a crucial responsibility: to find and preserve evidence. In the past, we simply pulled on a pair of gloves and maybe a mask to filter out odors, and got to work. Nowadays, we know that’s not enough to protect scenes from unintentional contamination of evidence. Contamination is a serious problem that can ruin evidence and jeopardize a criminal case. Attorneys for the defense and the prosecution will scrutinize the integrity of your evidence. You can expect questions about the methods used to collect and handle evidence when your case goes to court. While it may be impossible to eliminate contamination from every scene, you can prevent most of the problem by planning ahead and developing standard methods for working a scene.

As a CSO, your job is to preserve the scene as it is and to account for the evidence left by the suspect. Contamination occurs when something is introduced into the scene that was not previously there. Most contamination comes from the people investigating the scene. Remember, any time you walk in or out of the scene, you can bring something in or out with you. By taking a few precautions, you can avoid a lot of problems.

One of the most important ways to limit contamination is by limiting access to the scene. First, evaluate the scene. Consider what types of evidence are present and the best method for collecting them. Figure out if you’ll need additional staff or outside experts. Some evidence may need to be processed quickly and carefully because it’s fragile. At an outdoor scene, weather and the environment become factors. You may need a tent to protect the scene and to maintain privacy. Once you have an overall plan in place, you can control who enters the scene. Doing so decreases the chances of contamination. People who aren’t working the scene usually don’t need to be there. Often with a major crime, a variety of detectives and other officials will want to view the scene. But with the availability of smart phones and other devices, they don’t all need to be there; instead, you can use live stream videotaping to bring the scene to them. Do your best to keep the number of people at the scene to a minimum, and make sure you keep a log of everyone who enters and leaves the scene. Anyone at the scene can be subpoenaed, so an accurate record is critical.

In some cases, preventing contamination may be especially difficult. If multiple first responders are tending to the victim, their first responsibility is to the victim; they are not necessarily thinking about scene preservation. As a result, they may inadvertently leave fingerprints, footprints, and even DNA evidence behind. You must account for that evidence. Interview each responder and get a full report of their actions while at the scene. Get fingerprints, shoe print patterns, DNA, etc., from each person for elimination purposes.

Once you’ve established who will enter the scene, limit contamination by wearing the proper protective clothing. Tyvek suits, masks, gloves, and booties are essential. Cover your head to avoid dropping hair on the scene. Change gloves and booties frequently. Evidence is often on the floor; if you get blood or other evidence on your booties, change them.

Gloves can become contaminated in many ways. Whenever you touch blood or other fluids, change gloves. Also be alert to other sources of contamination. If you touch your eyes, scratch your nose, or cover your mouth when you cough, change gloves. If you pick up an item at the scene, change gloves before touching anything else. The first item might have the suspect’s DNA on it. If you touch the item and then grab something else, you could transfer evidence onto the second item. Every time you think your gloves have been compromised, change them. Consider wearing two pairs of gloves; when you need to change them, you can remove the top layer and quickly put on a fresh pair.

Collect booties in a paper sack and send them for processing in case they have any evidence on them. Dispose of gloves and other items in a bag labeled as biohazard. All items you dispose of should be bagged and documented, but not thrown away. Save these items until you’re certain they’re not needed.

A staging area can make it easier to protect your scene. Set up a tent outside the main crime scene for this purpose. This area gives officers a convenient place to change into booties, Tyvek suits, gloves, and masks. It also allows them do their work out of the view of the public and the press.

Contamination can also occur when dealing with prints and DNA evidence. DNA can be transferred from one crime scene to another through tools. Use new tweezers at each scene. Fingerprint brushes and powders can retain DNA, so always use a clean brush and new magnetic powder at each scene. If you don’t, DNA from an old scene could show up at the new scene. Once you process a sample, protect it from contamination. For instance, if you swab a sample to air dry and then cough over it, you’ve compromised your evidence.

Keeping contamination of evidence to a minimum is especially critical when investigating a homicide. Homicide crime scenes are ripe for potential DNA evidence. Collect as much evidence as possible. Even if DNA can’t be lifted from a sample in the present, new methods may be available in the future.

Eliminating contamination at a crime scene can make or break a case. It’s up to you to be diligent about protecting the scene and your evidence.

Dick Warrington is in research and development and a crime scene consultant and training instructor for the Lynn Peavey Company.