DNA Evidence: What Patrol Officers Need to Know

The advent of DNA analysis has revolutionized criminal investigations, and new techniques continue to improve the ways in which it can be used.

There's no question that forensic DNA evidence allows police to solve crimes that would otherwise go unsolved. Before its use, forensic biological science was limited to blood typing and other tests that couldn't identify an individual contributor of the genetic material. Now many people take for granted the ability to link a specific person to a crime scene.

As you probably know, DNA stands for deoxyribonucleic acid. Every person, except for identical twins, has a unique DNA profile or genetic fingerprint. Professor Sir Alec Jeffreys developed the scientific basis for DNA fingerprinting in the mid-1980s.

It's this unique fingerprint that allows forensic DNA to identify specific individuals that may have contributed DNA evidence to a crime scene. The advent of DNA analysis has revolutionized criminal investigations, and new techniques continue to improve the ways in which it can be used.


Of course, DNA is only useful in finding suspects if it can be matched to specific individuals. That's why the DNA Identification Act of 1994 provided the statutory authority to create a national DNA database. The FBI oversees the national forensic DNA program, which is called CODIS, the Combined DNA Index System.

There are two primary types of DNA profiles stored in the national DNA database. The first are convicted offender and arrestee profiles. Each state has its own laws regarding the arrest and conviction requirements that allow an offender sample to be uploaded into the national DNA database. Many states are passing laws to allow DNA profiles to be collected at the time of arrest, not after conviction. This is a good thing and will lead to many more cases being solved through DNA. The second type is forensic profiles. These are DNA samples that have been recovered from crime scenes and also from missing persons.

The CODIS Website provides striking statistics regarding the significance and success of this technology. According to an FBI administered Website, as of November 2009 the national DNA database contained more than 294,027 forensic DNA profiles and 7,635,428 convicted offender and arrestee DNA profiles. All 50 states have contributed to the database, which has assisted more than 100,000 investigations by providing law enforcement agencies with more than 101,900 hits or DNA matches.

Trace Evidence

DNA technology has evolved during the 13 years I've been a detective. First, the sample size required for DNA analysis has gotten smaller and smaller. No longer do we need to collect an entire drop of blood or large semen stain. Today, sample sizes are measured in nanograms of genetic material-just over a billionth of a gram is needed to generate a complete DNA profile. This minute sample size has allowed for the advent of trace or touch DNA collection.

Touch DNA is the process of collecting trace amounts of DNA from specific items of evidence. This technique is used in all types of cases, including property crimes. This is perhaps the most significant advancement in forensic DNA technology that I've seen during my career.

Solving Cases with Touch DNA

I worked a sexual assault a few years ago in which touch DNA helped solve the case. An unknown male had raped a young woman. There were no witnesses and no real physical evidence was recovered from the scene.

The victim's body and clothing is the primary crime scene in most sexual assault investigations, but the sexual assault examination didn't reveal the presence of semen on the victim's body. When the state forensic laboratory examined the sexual assault kit, only the victim's DNA profile was recognized-no male DNA was found. This was not surprising because the suspect had a difficult time maintaining an erection and he didn't ejaculate.

In this case the suspect had briefly kissed the victim's upper chest/lower neck area. Knowing this, a prudent patrol CSI swabbed the victim's neck with sterile cotton swabs moistened with distilled water. The officer feared that this transitory biological evidence would disappear once the victim put on clothing before being transported to the hospital. It turns out that these swabs provided the suspect's full DNA profile-the only piece of evidence in this case to do so.[PAGEBREAK]

DNA Mixture

Trace DNA collection poses certain challenges to law enforcement and forensic laboratories. Because the sample size is so small, the likelihood of collecting various people's DNA in one sample increases. These mixtures make it challenging for laboratories to resolve individual contributors.

To complicate matters, trace DNA often involves collecting DNA from objects that many people may have touched. It's not uncommon to develop DNA profiles from people (known or unknown) that have nothing to do with the crime. This can open the door for defense attorneys to argue an "alternate suspect" defense. Additionally, as the sample size shrinks the possibility of contamination is increased.

Therefore more emphasis must be placed on proper collection techniques and procedures, as well as minimizing scene contamination. This starts with the patrol officer.

First, Do No Harm

Law enforcement agencies differ. Some patrol officers routinely handle crime scene investigation and evidence collection, while others call in the crime scene unit on every case. Regardless of what end of the spectrum you fall on, the one thing we can all agree on is that we don't want to contribute our DNA at the scene.

I think about this every time I enter a scene, whether it's a burglary, sex assault, or homicide. The first rule of medical ethics is, "First, do no harm." I often think of this as I process crime scenes. My goal is to leave things as is by trying to minimize scene contamination. DNA contamination is a real concern.

Every scene is different, but some basic steps will help minimize contamination. Gloves are a must. Whether leather, latex, nylon, or nitrile, gloves will prevent you from depositing DNA on items in the scene. If you're not wearing gloves, minimize your contact with items within the scene; if you don't touch an object the odds are good your DNA won't show up on that object. Small things like sneezing could also transfer DNA, so this is something to think about.

Also, patrol officers are always first on scene and they have the authority to control everything within that area. Take active steps to ensure that all other people and personnel in the scene follow these guidelines. These are just a few of the matters you'll need to think about.

Who Handles It

The next question is whether the DNA evidence is best collected by the patrol officer or whether specialized crime scene technicians are needed. Or is the item of evidence best carefully packaged and delivered to the crime lab for DNA collection? There are no strict rules. I generally prefer that the item of evidence be collected and carefully packaged, if this is reasonable. This way the state lab can conduct trace DNA collection in a controlled setting with its specific validated techniques.

However, not all items can be packaged and delivered to the lab. Patrol officers will still need the basic tools and techniques to collect DNA evidence on scene.

The good news is that most biological evidence is collected in a similar manner-whether it's blood, semen, saliva, or possible trace DNA. I'm talking about DNA evidence that needs to be collected from a surface, not a cigarette butt or beer bottle discarded by the suspect. These items are just collected and packaged and the lab handles the rest.

Using Swabs

I'm also seeing a lot of patrol officers now collecting buccal (pronounced "buckle") swabs. These are cheek swabs that are used to obtain a person's DNA profile. Buccal swabs are collected from victims, witnesses, suspects, etc., either by gaining consent or via a court order.

Buccal swabs are easy to collect. I place four swabs (some folks use just two) inside the person's open mouth. I then vigorously rub the inside of the cheek to liberate skin cells from the cheek surface to the swab. There is no need to wet the swabs prior to collection, but they should be dried before being packaged.

The basic supplies needed for DNA evidence collection are latex or nitrile gloves, medical grade sterile swabs (individually packaged or two to a package), distilled water, clean envelopes, and packaging material. I also like swab boxes. These are small boxes that the swabs are placed into after collection. The boxes have air holes and areas in which to write the date and time of collection, case number, etc.

The procedure is fairly straightforward. Once an area is identified for collection-whether a blood stain, possible semen stain, or item for touch DNA-wet a minimum of two sterile swabs with distilled water (I usually use four swabs because I feel like you can't collect too much DNA). I prefer small individual ampoules of distilled water that are discarded after one use because this minimizes the potential for contamination.

Once the swabs are moistened, proceed to rub the swabs in the stain or on the area that you want to collect DNA from. Air dry the swabs, then package them in clean envelopes or swab boxes. Then place this inner packaging in another larger envelope that is sealed with evidence tape.

It's important to minimize cross-contamination by changing gloves and equipment for each sample that is collected. Once the item is collected it should be entered into property and evidence using local guidelines and protocols regarding refrigeration.

In many criminal investigations, DNA evidence is critical to both identifying the perpetrator and ruling out people that weren't involved in the crime. The national DNA database will continue to grow as more and more convicted offender and arrestee DNA profiles are entered into the system. This will lead to a significant increase in the number of DNA hits.

I also believe that patrol officers will continue to play a significant role in DNA collection as the technology becomes a larger part of police work. 

David Spraggs is a major crimes detective and certified bomb tech for the Boulder (Colo.) Police Department. He is a member of the POLICE Advisory Board and a frequent contributor.

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