Techniques A to Z

Forensic science techniques are the laboratory and field methods investigators use to test physical evidence: DNA profiling to identify a person, ballistics to link a bullet to a gun, toxicology to find drugs or poison, and roughly a dozen more. This page is an A to Z index of the major techniques named across Forensic Files, with a plain explanation of what each one does, the question it answers, and one honest limitation.

For the underlying ideas, what makes evidence admissible, how a lab avoids contamination, and why some methods hold up better than others, see Forensics Explained. For definitions of individual terms, the glossary covers them one by one.

Forensic science techniques, A to Z

The techniques below are the ones investigators reach for most often. Each entry says what the method is, the question it answers, and one honest limit on what it can show.

Ballistics, firearms, and toolmarks

Firearms examiners compare the microscopic marks a gun leaves on bullets and cartridge cases, viewed side by side under a comparison microscope, to judge whether a specific weapon fired a specific round.[1] Toolmark analysis extends the same idea to marks left by pry bars, knives, and bolt cutters. The question it answers is whether this gun or this tool made this mark. The honest limitation is that the match is a trained examiner's judgment rather than a measured statistic, and a 2016 review by the President's Council of Advisors on Science and Technology (PCAST) found the method had not yet met the standard for foundational validity.[2]

Bloodstain pattern analysis

Bloodstain pattern analysis reads the size, shape, and distribution of blood drops and spatter to reconstruct how a violent event unfolded: where a victim was, what kind of force was used, and the order things happened in.[3] It answers questions about position and movement that a body alone cannot. Its limitation is reliability between analysts: a National Institute of Justice study of 75 practicing analysts found their conclusions were wrong about 11 percent of the time and contradicted each other on roughly 8 percent of samples.[3]

DNA profiling (STR)

DNA profiling with short tandem repeats (STR) is the standard method for tying biological evidence to a person. A lab extracts the DNA, copies it with the polymerase chain reaction, measures the length of repeating sequences at 13 or more standard sites, and compares the resulting profile to a known sample.[4] With a full profile, the chance that an unrelated person matches at all 13 core sites is roughly one in a billion.[4] The main limitation is mixtures: when a sample holds DNA from several people, or only a trace amount, working out who contributed what gets much harder.

Entomology (forensic)

Forensic entomology uses insects, mainly blow flies that colonize a body within hours, to estimate how long someone has been dead.[5] Entomologists age the oldest larvae or pupae and work backward, using species growth rates and accumulated temperature (degree days), to a minimum time since death. It answers the time-since-death question in the window where a medical examiner's usual signs no longer apply. Accuracy leans heavily on reconstructing the temperatures the body actually experienced, and heat thrown off by masses of feeding larvae can skew the estimate.[5]

Fingerprints (latent prints and AFIS)

Latent print examination compares the ridge detail in a crime-scene print against a known print to decide whether both came from the same finger. Automated systems (AFIS, the Automated Fingerprint Identification System) speed the search by returning a ranked list of candidates from a database, which a human examiner then confirms by eye. PCAST judged latent print comparison to be foundationally valid, one of the few pattern methods to clear that bar, while noting it still carries a substantial false positive rate.[2] So a database hit is a strong lead, but the final call rests on human judgment that is not error free.

Forensic anthropology

Forensic anthropologists examine skeletal remains to build a biological profile: estimated age at death, sex, stature, and ancestry, plus any injuries and a rough time since death.[6] The question they answer is who the remains might be, which narrows a search against missing person records. Their output is a set of estimates and ranges rather than a name, so identification still depends on DNA, dental records, or other confirming evidence.[6]

Forensic odontology

Forensic odontology has two very different uses. Comparing a decedent's teeth against dental records and X-rays is a well-established way to identify remains, especially after fire or advanced decomposition. Bite mark comparison, which tries to match an injury on skin to a suspect's teeth, is far weaker: PCAST found no scientific basis for reliably identifying a person from a bite mark, and there is little agreement even among odontologists that it works.[2]

Gas chromatography and mass spectrometry (GC/MS)

Gas chromatography and mass spectrometry (GC/MS) identify unknown chemicals: illegal drugs, accelerants in a suspected arson, poisons in tissue. Chromatography separates a mixture into its parts, and the mass spectrometer records a spectrum for each part that gets matched against a reference library to name the compound.[7] Mass spectrometry is treated as a confirmatory, structural identification, which is why it is usually the lab's last word on what a substance is. Its limit is coverage: a compound can only be identified if a matching reference spectrum exists, and careful sample preparation matters.

Investigative genetic genealogy

Investigative genetic genealogy takes crime-scene DNA, reads hundreds of thousands of single-nucleotide markers (SNPs) across the genome, and uploads that profile to consumer genealogy databases to find relatives of the unknown source.[8] Investigators then build family trees to narrow toward a suspect, the method that identified the Golden State Killer suspect in 2018.[8] It generates a lead, not an identification: Justice Department policy limits it to violent crime and unidentified remains and bars an arrest based on the genetic association alone, with standard DNA testing required to confirm.[8] More recent cases built on it appear in Latest Breakthroughs.

Luminol and presumptive blood tests

Luminol is a presumptive test for blood: sprayed on a surface, it glows where it meets the iron in hemoglobin, revealing stains that have been wiped away or are invisible to the eye.[9] It answers a screening question, whether blood is likely present, and can surface traces even after cleaning. It is not proof of blood, though: bleach, some metals, and plant substances such as horseradish can make luminol glow too, so a positive result has to be confirmed with a more specific test.[9]

Mitochondrial DNA

Mitochondrial DNA (mtDNA) analysis is the fallback when standard DNA fails. Because each cell carries many copies, mtDNA can often be recovered from old bones, teeth, and hair shafts that hold no usable nuclear DNA.[10] It is inherited only from the mother, which answers questions about maternal lineage and lets investigators compare remains to a living maternal relative. That same trait is its weakness: everyone in a maternal line shares the same sequence, so mtDNA points to a family rather than a single person.[10]

Questioned documents

Questioned document examination looks at handwriting, signatures, ink, paper, and alterations to judge whether a document is genuine and who wrote it. A trained examiner compares features such as letter shapes and spacing against known samples. The comparison rests on visual judgment, and NIST has flagged human error and cognitive bias as real risks to the reliability of handwriting conclusions.[11]

Toxicology

Forensic toxicology screens blood, urine, and tissue for drugs, alcohol, and poisons, then usually confirms any finding with an instrument such as GC/MS. It answers whether a substance was present and roughly how much, which informs the cause and manner of death. Interpreting that number is the hard part: after death, drugs redistribute through the body, so a concentration measured in autopsy blood may not reflect the level at the time of death.[12]

Trace evidence (hair, fiber, and gunshot residue)

Trace evidence covers the small transfers left behind: hairs, textile fibers, and gunshot residue. Most of it is class evidence, meaning it can say two samples are consistent but cannot single out one source. Microscopic hair comparison, for instance, cannot identify a specific person, and an examiner cannot state the odds that a hair came from someone chosen at random;[13] an FBI and Justice Department review found examiners had overstated the strength of hair matches in nearly every case it examined.[14] Gunshot residue (particles of lead, barium, and antimony read under an electron microscope) shows someone was near a firing gun, but the particles transfer easily by contact, so their presence does not prove a person pulled the trigger.[15]

How reliable are forensic techniques?

Reliability varies a lot from one technique to the next, and it helps to separate the methods grounded in measurable chemistry and biology from the ones that rest on an examiner's eye. DNA profiling, toxicology, and chemical identification by GC/MS produce results that can be checked and repeated. Pattern-comparison methods (firearms, bite marks, and some handwriting and bloodstain work) depend more on trained judgment, and studies have now put real error rates on them.[3]

A 2009 National Academy of Sciences report questioned the scientific footing of many of these pattern methods and pushed the field toward measured error rates and validation. The 2016 PCAST report went further, judging only a handful (single-source and simple-mixture DNA, plus latent fingerprints) to be foundationally valid at the time.[2] None of this makes the techniques worthless. It means an honest examiner states what a method can and cannot show, which is the same lens worth bringing to the cases in the episodes. For plain-language definitions of any term above, see the glossary.

Sources

1. National Institute of Standards and Technology, Firearm Examination: A NIST Scientific Foundation Review
2. National Institute of Justice, Post-PCAST Court Decisions Assessing the Admissibility of Forensic Science Evidence (summarizing the 2016 PCAST report)
3. National Institute of Justice, Study Reports Error Rates for Bloodstain Pattern Analysis (2021)
4. National Institute of Justice, What Is STR Analysis?
5. S. Matuszewski, Post-Mortem Interval Estimation Based on Insect Evidence: Current Challenges, Insects (2021)
6. National Institute of Justice, Overview of Forensic Anthropology
7. National Institute of Standards and Technology, Comprehensive Data Evaluation Methods Used in Developing the SWGDRUG Mass Spectral Reference Library for Seized Drug Identification
8. National Academies of Sciences, Engineering, and Medicine, Law Enforcement Use of Probabilistic Genotyping, Forensic DNA Phenotyping, and Forensic Investigative Genetic Genealogy Technologies: Proceedings of a Workshop (2024)
9. F. Barni et al., Forensic application of the luminol reaction as a presumptive test for latent blood detection, Talanta 72 (2007) 896-913 (PubMed record)
10. Mitochondrial DNA in Human Identification: A Review, PeerJ (2019)
11. National Institute of Standards and Technology, Forensic Handwriting Examination and Human Factors: Improving the Practice Through a Systems Approach
12. Difficulties Associated with the Interpretation of Postmortem Toxicology, Journal of Analytical Toxicology (2024)
13. The Current Status of Microscopical Hair Comparisons, The Scientific World Journal (2001)
14. PBS NewsHour Weekend, Report: FBI investigators overstated evidence against criminal defendants (2015)
15. INTERPOL Review of Gunshot Residue 2019 to 2021, Forensic Science International: Synergy (2022)