DNA profiling using short tandem repeat (STR) analysis is the technique most likely to be used by forensic scientists. It is the industry standard for identifying individuals from biological evidence found at crime scenes, and it forms the backbone of criminal databases worldwide. While forensic science encompasses many methods, from fingerprint comparison to toxicology screening, STR-based DNA profiling stands apart as the most widely applied and relied-upon identification technique in modern casework.
How STR DNA Profiling Works
Your DNA contains short, repeating sequences scattered across your chromosomes. These repeating segments, called short tandem repeats, vary in length from person to person. By measuring the number of repeats at multiple locations in the genome, forensic scientists can build a genetic profile that is essentially unique to one individual.
The process follows a consistent sequence. First, DNA is extracted from a biological sample, which could be blood, saliva, skin cells, or hair roots. Scientists then measure how much DNA is present and amplify specific STR locations using a copying technique called PCR. The amplified fragments are separated by size on an automated instrument called a genetic analyzer, and bioinformatics software reads the results. The entire process can produce findings on the same day, and it works with as little as 0.5 to 1 nanogram of DNA, a quantity invisible to the naked eye.
The FBI’s Combined DNA Index System (CODIS) requires analysis at 20 core STR locations for profiles uploaded to the national database. The original standard set 13 loci, but seven additional markers were added in 2017 to further reduce the chance of coincidental matches between unrelated people. Profiling at this many locations makes the probability of two unrelated individuals sharing the same profile astronomically small.
Why DNA Profiling Became the Gold Standard
STR analysis replaced an older method called restriction fragment length polymorphism, which required larger, higher-quality DNA samples and took much longer to complete. STR typing works on degraded or partial samples, the kind forensic scientists routinely encounter at crime scenes where evidence has been exposed to heat, moisture, or time. The fragments being analyzed are relatively small (80 to 400 base pairs), which means even broken-down DNA often still contains usable information.
The technique also allows multiplexing, meaning scientists can examine multiple STR locations simultaneously in a single reaction. This saves time, conserves limited evidence, and produces a highly discriminating profile from one test run.
Fingerprint Analysis
Fingerprint comparison remains one of the most common forensic techniques and predates DNA profiling by over a century. Modern systems use Automated Fingerprint Identification Systems (AFIS) that digitize prints and compare them against massive databases. The software identifies small features in a fingerprint pattern called minutiae, such as ridge endings and bifurcations, and calculates whether two prints share enough of these features in the same positions and orientations to be considered a match.
The matching algorithm measures the physical distance and angle between corresponding minutiae on two prints. If both values fall below set thresholds, those minutiae are considered paired. The system then ranks candidates by a similarity score based on the total number of paired minutiae. A trained examiner reviews the top-ranked candidates on a high-resolution monitor to make the final determination. Fingerprints are powerful when a clear print is available, but partial or smudged prints limit their usefulness.
Forensic Toxicology
When the question involves what substance is in someone’s body rather than who was at the scene, forensic scientists turn to toxicology. The primary tool here is mass spectrometry coupled with chromatography, most commonly gas chromatography-mass spectrometry (GC-MS). This combination first separates a complex mixture into its individual components, then identifies each one by its molecular structure.
GC-MS can screen for dozens of drugs in a single run. One validated method identified 54 different substances in urine samples, including cannabis metabolites, cocaine, hydrocodone, and sedatives. More advanced versions of the technology can detect cannabinoids in hair at concentrations as low as 0.007 nanograms per milligram, well below the cutoff values set by professional toxicology organizations. This sensitivity makes it possible to detect drug use days or weeks after exposure.
Gunshot Residue Analysis
When investigators need to determine whether someone fired a weapon, they collect samples from hands or clothing and examine them using scanning electron microscopy paired with energy dispersive X-ray spectrometry (SEM/EDS). This technique identifies tiny particles produced when a gun is fired by analyzing both their shape and chemical composition.
The hallmark of gunshot residue from standard ammunition is the presence of lead, antimony, and barium together in a single particle. These three elements originate from the primer compound in the cartridge. Additional elements like silicon, calcium, aluminum, copper, and tin may also appear. Some specialty ammunition produces different signatures: certain lead-free primers generate particles containing titanium and zinc instead. The ability to pinpoint the elemental makeup of individual microscopic particles gives SEM/EDS a high level of confidence in confirming gunshot residue.
Ballistics Comparison
Forensic scientists can link a fired bullet or cartridge case to a specific weapon by examining the unique marks a firearm leaves on ammunition. The Integrated Ballistic Identification System (IBIS) automates this process for cartridge cases. A fired casing is placed in a holder under a microscope, and the system separately photographs the breech face impression and firing pin impression at high magnification.
The software extracts a digital signature from each image, then searches the database in two phases. First, it screens all casings of the same caliber and firing pin shape, ranking them by similarity. Then the top 10% undergo a more detailed comparison where the software rotates the images through 360 degrees to find the best possible alignment. An examiner reviews the highest-scoring candidates side by side on screen. This process can connect cases across jurisdictions by linking cartridge cases recovered at different crime scenes to the same weapon.
Digital Forensics
As crimes increasingly involve smartphones and computers, digital forensic extraction has become a routine part of investigations. Analysts recover data from mobile devices using either logical extraction, which pulls accessible files and databases, or physical extraction, which copies the entire contents of the device’s storage. Modern smartphones with full-disk encryption have pushed forensic methods toward bypassing security features and exploiting system vulnerabilities to access locked devices.
Next-Generation Sequencing for Complex Cases
Standard STR profiling has limits. It struggles with DNA mixtures from multiple people, heavily degraded samples, and cases where two alleles happen to be the same size but have different sequences. Next-generation sequencing (NGS) technology addresses these gaps by reading the actual DNA sequence rather than just measuring fragment length, making it possible to distinguish alleles that look identical under traditional methods.
NGS is particularly valuable for mixed DNA samples. In one study, researchers detected a minor contributor’s DNA even when it was present at a ratio of just 1 to 250 compared to the major contributor. With deeper sequencing, ratios as low as 1 to 1,000 may be detectable. The technology also improves mitochondrial DNA analysis, which is used when nuclear DNA is too degraded, such as in skeletal remains or hair shafts without roots. NGS can detect variations within a person’s own mitochondrial DNA across different tissues, adding another layer of discriminating power. While not yet as universally adopted as STR profiling, NGS is increasingly used in cases where conventional methods fall short.