Law enforcement gathers DNA from persons of interest through several methods, ranging from a simple cheek swab during booking to collecting a discarded coffee cup without the person ever knowing. The method used depends on the legal situation: whether someone has been arrested, whether a warrant has been obtained, or whether the person voluntarily or unknowingly left biological material behind.
Direct Collection: The Buccal Swab
The most straightforward method is a buccal swab, which collects cells from the inside of the cheek. An officer or technician places a foam-tipped applicator inside the person’s mouth and rubs it firmly against the inner cheek for at least 10 seconds per side, pressing hard enough that the outline of the swab is visible from outside the cheek. The process is repeated on the opposite cheek. Some protocols use a pair of sterile cotton swabs twisted simultaneously against the cheek lining instead of a foam applicator.
This type of collection typically happens at booking after an arrest for a serious offense. The U.S. Supreme Court ruled in Maryland v. King that taking a cheek swab from someone arrested on probable cause for a serious crime is reasonable under the Fourth Amendment and does not require a separate warrant. For individuals who haven’t been arrested, investigators generally need either a court order, a warrant supported by probable cause, or the person’s voluntary consent before swabbing.
Collecting “Abandoned” DNA
When investigators can’t get a direct sample, they often turn to items a person has discarded in public. Coffee cups, drinking glasses left at a restaurant, cigarette butts, tissues, and chewing gum are all common sources. The legal principle here is straightforward: once you leave a biological sample on an item in a public place and walk away from it, you have no expectation of privacy in that material. Courts have consistently held that police seizure of these discarded items does not trigger Fourth Amendment protections.
This approach, sometimes called surreptitious collection, allows investigators to build a DNA profile without the person of interest ever being aware. It is particularly useful early in an investigation when there isn’t enough evidence for an arrest or a warrant but a DNA comparison could confirm or eliminate a suspect. An officer might follow a person of interest to a café, wait for them to leave, and then bag the cup they drank from.
What Biological Material Contains Usable DNA
DNA profiling requires cells that contain a nucleus. The most common sources in forensic work are liquid or dried blood, saliva, semen, hair with the root follicle still attached (a hair shaft cut or broken off without the root generally won’t work), bone, teeth, and loose skin cells. Skin cells, also called epithelial cells, are shed constantly and can be found on virtually anything a person has touched or worn.
The minimum amount of DNA needed to generate a usable profile is surprisingly small. Most standard profiling systems require between 0.1 and 0.5 nanograms, which is a fraction of a billionth of a gram. “Trace DNA,” the kind left behind by a simple touch, refers to quantities below 0.1 nanograms. Some labs set their working threshold at 0.2 nanograms, below which results become less reliable. But advances in amplification technology have pushed the lower limit even further, with some specialized kits producing results from as little as 0.125 nanograms of degraded or damaged material.
Touch DNA: Recovering Traces From Surfaces
Touch DNA collection focuses on the invisible skin cells a person leaves behind on objects they’ve handled: a doorknob, a steering wheel, a weapon grip, a piece of clothing. Several techniques exist, each suited to different surfaces.
The most common approach is the double-swab technique. A wet swab is rubbed across the surface first, followed immediately by a dry swab. The moisture loosens cellular material, and the dry swab picks up what remains. This sequential method recovers more DNA than a single swab alone. Traditional cotton swabs tend to trap biological residue within their fibers, reducing how much material actually makes it to the lab, so newer synthetic swabs are increasingly preferred.
For porous or rough surfaces like brick, an industrial device called the M-Vac, originally designed to sample food for pathogens, uses a wet-vacuum approach that outperforms double-swabbing. Adhesive tape lifting is another option for solid surfaces: tape with strong adhesion is pressed against the area and peeled away, pulling skin cells with it. This method is fast and can yield more DNA than swabbing, though the sticky residue complicates lab analysis. On soft materials like fabric, analysts sometimes cut out the relevant section entirely. Cutting produces the highest DNA recovery of any method, but it’s irreversible and obviously destroys part of the evidence.
Investigative Genetic Genealogy
When a crime scene DNA profile doesn’t match anyone in law enforcement databases, investigators increasingly turn to genetic genealogy. This process involves generating a detailed genetic profile from the crime scene sample and uploading it to public genealogy databases that allow law enforcement use, currently GEDmatch PRO, FamilyTreeDNA, and DNASolves.
The database search identifies people who share segments of DNA with the unknown sample, meaning they inherited those segments from a common ancestor. These matches are typically distant relatives: first cousins or beyond. Algorithms measure the size and frequency of shared DNA segments to estimate how many generations back two people’s family trees converge. Investigators and genealogists then build out family trees using public records, birth and death certificates, and other genealogical tools, working forward through generations until they identify a candidate who fits the right age, location, and circumstances. At that point, investigators usually collect an abandoned DNA sample from the candidate to confirm or rule out a match.
Processing and Database Entry
Once collected, DNA samples follow one of two processing tracks. Traditional laboratory analysis sends the sample to a forensic lab, where technicians extract the DNA, amplify it, and generate a profile based on short tandem repeats (specific repeating patterns at known locations in the genome that vary between individuals). This process can take days to weeks depending on the lab’s backlog.
Rapid DNA technology has compressed that timeline dramatically. Fully automated instruments at booking stations can produce a DNA profile from a mouth swab in one to two hours with no lab or human review required. Under the FBI’s Rapid DNA initiative, a qualifying arrestee’s profile is enrolled in the national database (CODIS) during booking and searched against unsolved crimes within 24 hours. For crime scene evidence rather than booking swabs, a lab analyst must still review the Rapid DNA output before it can be uploaded.
The national database, CODIS, accepts profiles in specific categories: convicted offenders, arrestees, detainees, forensic casework samples from crime scenes, unidentified human remains, missing persons, and relatives of missing persons. One important distinction: a sample taken directly from a suspect is classified as a “deduced suspect sample” and is not eligible for upload to the national index. It can only be compared against existing profiles, not stored alongside them.
Preserving the Sample
How a DNA sample is handled after collection matters as much as how it’s gathered. Biological evidence degrades when exposed to heat, moisture, and bacteria. The two most effective preservation methods are air-drying at room temperature and freezing at around negative 30 degrees Celsius. Both approaches consistently produce higher DNA quantities and more complete genetic profiles than alternatives like submersion or chemical preservation. Forensic protocols call for drying swabs and evidence before sealing them in paper packaging (not plastic, which traps moisture and accelerates degradation) and maintaining an unbroken chain of custody documenting every person who handled the evidence and when.