Forensic Science

Notes by Rob James, 2025

PART I. CRACKING THE CASE

1.    FORENSIC SCIENCE

Forensic science is the application of scientific disciplines to law. The main elements are crime scene investigation; evidence analysis; scientific, psychological and legal assessment; and reports and testimony.

Locard’s Exchange Principle: “every contact with another person, place or object results in an exchange of evidence.” (Edmond Locard established the first forensic laboratory in Lyons, 1910. Sherlock Holmes fictionally studied differences in types of ash, but years later Locard produced a genuine study to that effect.)

“CSI Effect”: (1) juries watch CSI, and now expect DNA evidence and other scientific proof, which isn’t always possible; (2) criminals watch CSI, and now wear gloves more often; and (3) even future victims watch CSI, and pull out their own hair strands and spit in the abduction car.

Expert testimony must be qualified in federal court (per Daubert (U.S. 1997)) or state court (per Daubert or Frye (D.C. Cir. 1923)). Circumstantial evidence is often more reliable than witness identification, forced confessions, and other kinds of direct evidence. A “false positive” (Type I) error tends to implicate the innocent, and is worse than a “false negative” (Type II) that tends to exculpate the guilty.

2.  CRIME SCENE INVESTIGATION

Secure the crime scene and auxiliary scenes. Make the site safe, provide medical assistance, preserve scene and evidence, wear personal protective equipment. Detain the reporters of the crime and keep them from the scene (they may be complicit; and keep in mind that the criminal may have staged the scene). Detain other suspects and witnesses, and keep them separate from one another. Maintain security log and chain of custody log. Document systematic walk-though, take notes, sketches and photos (with reference-scale items; a rotating camera can create a digital twin of the scene).

One of my favorite scenes in True Detective season 1 is Marty Hart warning Rust Cohle not to jump to conclusions—“You attach an assumption to a piece of evidence, you start to bend the narrative to support it, prejudice yourself.” Francis Bacon’s version of science parallels Marty’s position—that one should begin with pure observations, without any theorizing whatsoever. But as Karl Popper contended and as my philosophical compatriot Rust would concur, the ideal scientist must formulate hypotheses on the go, based on the facts as they are emerging. The key is constantly to test those hypotheses, and all alternatives, against the accumulating evidence. Popper and Bacon on observation This reminds me of Werner Sombart’s concept “No theory, no history,” which annoyed me as an undergraduate but whose truth has been revealed to me over time.

Sorry, where was I? Oh, yes, be sure to get a search warrant. A warrant requires presenting to a judge an affidavit identifying probable cause, the place to be searched, and the items to be searched and searched for. Make the list very broad, and include small-sized things and electronic records so you can look everywhere. No warrant may be necessary if emergency, if risk of impending loss, if lawful arrest (can search items in immediate possession) or if consent.

Where is the corpse, if it’s missing? Look downhill, look for variations in terrain, use dogs and aerial surveys, look for decay (heat, presence of nitrogen), use electric conductivity for bodies and magnetic sweep for buried metals. For the Oklahoma City bombing debris, even specially trained blowflies were released to aid in the grim and grisly search.

3.   THE CRIMINAL MIND

Psychological inquiry is relevant first to motive, in order to identify suspects, and then to mens rea (criminal state of mind), in order to convict them. Forensic psychiatry (noncooperative subject, you are trying to get truth and convictions) is contrasted with clinical psychiatry (cooperative subject, you are trying to help patient).

First review medical history, including drug history, of suspect or witness. Can conduct psychological exams, including personality inventories (MMPI, MCMI, CPI), projective testing (Rorschach, TAT), and Adult IQ tests (Wechsler WAIS). Begin interrogations non-confrontationally. Hypnosis and sodium pentothal are disfavored these days, as the examinee becomes overly susceptible to suggestion.

Criminals and non-criminals both lie. Watch for exaggeration in opposite direction of crime (“I respect women”), malingering, nervousness, body language, eye movement, micro-expressions.

Distinguish competency to stand trial (understand charges, consequences, roles of judge and lawyers) from sanity to be criminally liable (Daniel M’Naghten case (1843) and Omnibus Crime Code standards). General inquiry is whether the defendant appreciates nature and consequences of actions, regardless of whether he appreciates their illegality. Diminished capacity applies to temporary impairments and can reduce or replace punitive sentences.

Serial offenders often have no relation to victims. Distinguish mass murderer (>3 all at a time), spree killer (>1 in separate occasions over a short time), and serial killer (>3 but over a longer time). Antisocial Personality Disorder (ASPD) profile either a psychopath (calculating, manipulative) or a sociopath (impulsive). They often have a specific fantasy of a target or a method, enacted and re-enacted.

“Profiling” is derived from techniques of the FBI Behavioral Science Unit, now NCAVC. “MO” or modus operandi is the set of tools and strategies used to commit the crime (L.W. Atcherly’s ten dimensions). “Signature” in contrast is a trait or method other than the commission of the crime itself that is more personal to the offender, something he is particularly attached to. What is his hunting range? Link of geography of crimes to geography of suspects in VICAP database. Profile the victims too.

PART II. ANALYZING EVIDENCE

The FBI Handbook covers over fifty types of evidence that can be analyzed, including audio recordings and bird feathers.

4.  FINGERPRINTS

More broadly, this category includes “friction ridges” on fingertips, toes, palms and soles (even the prehensile tails of our cousins the monkeys). Alphonse Bertillon measured many adult features like head sizes and took mug shots, but most of these data proved useless; by 1910 fingerprints won out. Francis Galton 1892 categorized arches, loops and whorls. Need prints of fingers on both hands; they had the Mona Lisa thief’s left-hand prints on file, but he left a right-hand impression. FBI has huge IAFIS database.

5.   FLUIDS

Blood is literally thicker than water; it is a composite of red and white blood cells, platelets and proteins. Plasma is the clear fluid of unclotted blood, serum is the yellow fluid of clotted blood. Blood clots within 3-15 minutes (subject to diseases and blood thinners). Assess pattern of bloodstains (arterial bleeding produces rhythmic impressions; a perpendicular drip produces a circle; a violent blow produces an ellipse, either or both a front spatter and a back spatter). Blood is not itself fluorescent, but luminol with hydrogen peroxide will react with red blood cells and that combination will show up under ultraviolet light. Other bodily fluids (you know them) will fluoresce by themselves or aided by other chemicals.

6.  IMPRESSIONS

Shoeprints can be patent (in stable soil), plastic (e.g., in snow or mud), or latent (e.g., on wood, cardboard or glass). Assess make and size of shoe, gait, weight, etc. Similar methods exist for tires, tools, fabrics.

7.  ARSON

Identify point of origin, ignition source, accelerant.

Why would someone commit arson? Insurance; cover tracks of some other crime; fraud; psychological deviance; revenge; murder; terrorism.

It takes 1500 F for two hours to completely incinerate a corpse; a typical fire is 500 F for much less time. It is thus a poor way to dispose of a body if you were inclined to do so.

Match heads have unique diatoms from powdered rock coming from different quarries. So a fire can be traced at least to a brand of matchbook in some cases.

PART III. THE BODY

8.   CAUSE AND MANNER OF DEATH: THE AUTOPSY

Determining fact of death—formerly stoppage of function of heart and lungs, now judged by brain death (EEG). Death can be natural, accidental, suicidal, or homicidal. The medical examiner (ME) is called on to do autopsy if trauma, unusual, sudden, or unexpected. The coroner is to receive a report in these and some other cases.

Over 50% of hospital deaths resulted in autopsies in 1946; they are now performed in fewer than 5%. To perform an autopsy, transport body to morgue in sheets zipped into body bag. Remove body carefully and retrieve any trace evidence. Measure height and weight, ascertain sex, age, race, hair and eye color. Look for distinguishing marks. Review trace evidence. Estimate time of death (rigor mortis, lividity, decay). Perform X-rays to view trajectory of gunshot wounds. Assess injuries (lacerations, contusions, stabs, gunshot entry, exit and bullets or shot remaining in the body). Dissect body, making the famous Y abdominal incision. Assess state of abdomen, stomach, other organs, sometimes commissioning toxicology tests for organs and tissues. Hair may show presence of arsenic. Brain inspection. Then return organs and suture up for the funeral.

9.     UNKNOWN BODY

Famous equation that one week in the open has body damage equivalent to two weeks in the water or eight weeks in the ground.

With a complete skeleton, one can be certain of gender. Male has taller and narrower pelvis, narrower pelvic outlet, narrower sciatic notch (“bay” on rear of pelvis), thicker ribs, curved posterior ramus of mandible (back of jaw), brow ridge.

Can judge age within 5-10 years: 20 baby teeth vs 28 adult teeth by age 12 and 4 wisdom teeth by age 18; skull sutures; pelvis keeps fusing till age 50; ribs get more pitted and sharper; bone density. Can gauge height within 1.5”, maybe race.

10.   TIME OF DEATH

Many so-called “clocks,” no one of which is very reliable. One medical examiner remarked that if one of the methods happens to get the timing correct, it is more the result of chance than of science.

Beware, this chapter gets gruesome. 

Body temperature starts at 98.6 F. Goes down about 1.5 F/hour until consistent with ambient temperature. That rate also depends on the ambient temperature itself, obesity, age, clothing.

Rigor mortis. First the body relaxes, But death stops production of ATP and therefore of ADP; cells no longer move calcium ions, causing muscles to contract and body to go rigid, starting in 2-6 hours. First the face, then extends toward extremities; 8-12 hours until full stiffening; then 12-24 hours of rigid stage. After a total of 48 hours, flaccid again.

Lividity. Stagnant blood follows gravity, so whatever is low turns grey-blue. But lower parts that are up against some surface (including tight clothing, bra strap) squeeze blood vessels down and are pale. If red skin, look for presence of poison like carbon monoxide or cyanide, or for extreme cold. If really purple skin, that’s fresher blood, evidence of heart failure or asphyxiation. Lividity sets in within 30 minutes to 2 hours, at a maximum for 8-12 hours.

Decay. First comes “autolysis,” self-digestion from contents of the body itself; internal enzymes eat away at inner tissues. Then comes “putrefaction” from environmental bacteria and yeasts. After 36 hours, greenish abdomen. Gaseous bloating from bacterial digestion starts in the face (protruding eyes (cloudy within 2-24 hours) and tongue), then skin blisters. Marbling of flesh as red blood cells break down and spread out along blood vessels. Abdomen swells next, and skin, hair and nails start to slough off. (If you pull on an arm or leg at this point, the skin may come off.) Green-black putrefying fluid, everything starts to break open. Organs decay in order, starting with lower intestines and proceeding to liver, lungs, brain and kidneys. The last to go are the stomach (because of resistant acids), uterus and prostate.

Can have mummification in hot, dry climate, adipocere process (turning the body into “soap”) in humid climates. A corpse tossed into water is initially a “sinker.” As the gases form during decay, it becomes a “floater.” Can take days in warm water, weeks or even months in cold water, for the body to surface. The University of Tennessee Body Farm demonstrates the timing of decay from various field conditions.

If the stomach has food, death within last few hours. If not but the small intestine has waste products, death within last 24 hours. If not but the colon has waste, death within maybe last 48 to 72 hours.

Entomology. Meguin’s classic treatise La faune des cadavres (Corpse Animals, 1893). Insects come in, first blowflies with super senses of smell; they appear within an hour out in the open. Female deposits eggs, maggots born within 24 hours, hard-shelled pupae form within 4-6 days, new flies emerge within 10 days, they immediately have sex, and the new females deposit eggs 2 days after that. Blowflies are followed by other flies, beetles, moth larvae, and hair-eating mites. Maggots ingest blood and can provide human DNA if the body is missing.

Hey, I told you this chapter was gruesome.

11.   TRAUMA

Gun shot wounds (GSW). Instant death only if shot in brain, heart or upper spinal cord; otherwise, death by exsanguination (bleeding out) or infection.

Assess entry wound to gauge distance, caliber, velocity, angle, and likely exit if there is an exit (if the bullet or shot is not still lodged in the body).

Strangulation produces bruising, fractured cartilage, and petechiae (hemorrhage spots in face and eyelids).

If shot from close, the entry wound is a small hole, with abrasion. Will have smudging from gases or burnt gun powder. If real close, will find unburnt gun powder and explosive gases inside body. If gun was pressed against skin, the entry wound will be a star pattern instead.

Assess exit wound, which will be larger, especially from soft bullets taking lots of tissue with them as they expand.

Stabbing (deep tissue), chopping (axe), slashing (incision or cut across skin and shallow tissue) wounds. Can be suicidal, homicidal, defensive, or accidental.

Blunt force trauma: sliding abrasion (pavement, rope), stamp abrasion (baseball bat), and patterned abrasion (chain, tire treads). Scab will start healing within 6-24 hours, and be finished in 12-20 days. Similar observations for contusions (bruises), hematomas (subdural blood buildup), and fractures.

Bites can match to teeth of criminal, as with Ted Bundy, though that exercise is somewhat unreliable. Head trauma includes epidural, subdural and intra-cerebral.

Thomas Edison was right—AC current is especially dangerous for electrocution, much like a heart attack.

Rape kit sperm test can detect motile or intact sperm within 12-24 hours. Beyond that, the tails drop off, but heads can still be detected for 7 days. Vaginal fluid can harbor sperm that long too. Sperm last longer in corpses because vaginal fluid has natural spermicides.

12.   ASPHYXIA

Normal air is 21% oxygen. At 11-15%, judgment is off. 10% unconsciousness, 8% death.

Suffocation (no O2), strangulation, drowning, toxic chemicals all prevent O2 uptake (even in open air). Asphyxiants include carbon monoxide (most common chemical suffocation), cyanide, and hydrogen sulfide.

PART IV. THE CRIME LAB

13.   SEROLOGY

Blood has solids like white blood cells (leucocytes), red blood cells (erythrocytes), platelets for clotting, enzymes, proteins. And plasma.

ABO blood types O 43%, A 42%, B 12%, AB 3%. Rhesus factor positive or negative. A+ means the blood has the A antigen and the Rhesus D factor, but does not have the B antigen.

14.   DNA

Adding the enzyme polymerase to small amount of DNA can grow more. Polymerase chain reaction (PCR) greatly expanded use of DNA.

Chromatography can help separate DNA (and other substances of forensic interest—drugs, poisons, fire starters, explosives) from other substances.

Watch for bias. Because African-Americans are disproportionately logged in arrest databases, familial DNA searches perpetuate and augment the bias.

15.   TOXICOLOGY

Digitalis in small concentrations can save lives, but in large concentrations kills. As Paracelsus wrote back in 1538, “the dose makes the poison.”

Alcohol: 0.08 BAC means 0.08% concentration of alcohol in blood by volume (grams per 100 milliliters). That is the general legal limit for driving; 0.03% is about one drink. 0.12% nausea and vomiting, 0.25% coma, 0.40% death. Breathalyzer 1954.

Opiates: natural (morphine, codeine), semisynthetic (heroin is morphine plus acetic anhydride or acetyl chlorate, which eventually breaks back down into morphine).

Hypnotics: barbituates (phenobarbital). Alcohol and barbituate combination suppresses respiration.

Stimulants: amphetamines (crystal methamphetamine), cocaine (breaks down inside body into different chemicals).

Hallucinogens: marijuana (active ingredient tetra hydro cannabinol (THC) typically 2-6%, but in hashish oil extract it’s 12%); peyote (cactus) buttons (mescaline), mushrooms (psilocybin). Fun fact, THC breaks down into a chemical detected in urine, giving rise to the drug test; you can fail such a test with “secondary smoke” simply by hanging around marijuana smokers.

Synthetics LSD, PCP (angel dust).

Date rape drugs Rohpyphol, Ecstasy, GHB, Ketamine. (Contrary to Hollywood, chloroform is not instant.)

Sniff airplane glue and other volatile organic chemical fumes. Anabolic steroids.

Poisons: cyanide, strychnine (a painful way to go), mushrooms (death caps, death angels), ethylene glycol (antifreeze), oxalic acid, metals (arsenic, mercury, lead), insulin. Legal execution often uses succinyl choline.

16.   TRACE EVIDENCE

Hair characteristics. Color, curly/straight, oily/dry, head/pubic. Racial differences not terribly reliable.

Glass. Entry is small, exit is large. If two bullets, you can tell from the web of cracks which shot came first.

Tests exist to examine fibers, whether natural, manufactured (from natural cellulose) or synthetic. Comparable tests exist for paint chips, soil, and plants (including pollen and seeds).

17.   FIREARMS

Stonewall Jackson was hit by a 0.67 caliber ball bullet (CSA), not a 0.58 miné ball bullet (USA), proving he was killed by friendly fire.

Handguns: revolver (bullet and shot cartridge (primer, gunpowder) go into cylinder, but spent shell casing remains in the gun while the cylinder revolves) versus pistol (magazine or clip, spring loaded, and each shell casing is ejected).

Rifles (lever or slide bolt to eject the cartridge. Handgun and rifle bores have rifling (interior lands and grooves).

Shotguns. Shells with pellet shot, not bullets. Size ranges from #20 to #12 to 0.33 for #00, called “double aught” shotgun.

Lead bullets are soft for lowest velocity, small caliber, .22 and .25 rifles. Semi jackets expose the soft lead at front for lower velocity but higher caliber like .357 and .44 handguns and rifles. Full metal or Teflon jackets for high velocity, high caliber, high-powered arms like magnums that penetrate bullet-resistant vests.

Each gun manufacturer has a unique grooving. .32 Smith & Wesson has 5 lands and grooves, clockwise; .32 Colt 6 lands and grooves, counterclockwise; .32 Browning, 6 lands and grooves, clockwise.

What you find is more likely exhaust gases, not unburned powder or residue of burned powder.

If serial numbers are filed off, there are deeper impressions that can be recovered.

18.   DOCUMENTS

Many techniques exist for reviewing paper, signatures, ink. Also techniques for analyzing computer use, social media, GPS data.

PART V. “THE TENS”

19.   TEN FORENSIC CASES

The books have interesting details about the Lindbergh kidnapping, Sacco/Vanzetti, Ted Bundy, and others. Jack the Ripper in 1888 Whitechapel was associated with five murders, which were separated from seven other apparently similar crimes due to the killer’s unique “signature” features. Hitler’s “diaries” were written on paper invented in 1954.

20.   TEN HOLLYWOOD UNREALISTIC FEATURES

Quick death (shoot or stab, bad guy is instantly dead): only if brain, heart or upper spinal cord. Otherwise bleed out. The physician examining the body of Julius Caesar opined that of the 23 stab wounds, only the one between the first and second ribs was fatal. Otherwise he might have staggered around like William Shatner or Sonny Corleone.

Pretty death (body still attractive when found): discolored, smelly, gruesome very quickly.

Bleeding corpse: clotting actually starts within 15 minutes.

Exact time of death (“she died at 10:30 pm”): you can just estimate a range over several hours or days of uncertainty.

One-punch knock out: you would be extraordinarily lucky to hit someone so unconscious for any length of time; look at a boxing match.

Disappearing black eye: takes three weeks minimum to heal. That is one of the many things that makes the movie Chinatown so impressive, the fact that a big star like Jack Nicholson would wear a nose bandage for so many scenes.

Poison or chloroform works instantly: except nerve gas, they all take some time.

Untraceable poison on the one hand or immediately detected on the other hand: all can be discovered, but ascertaining any of them takes some time.

Instant athlete (out of shape detective or hero runs for several city blocks): unrealistic unless it’s “Marathon Man.”

High technology laboratory: real crime labs are low-tech, dingy places.

Bibliography

FBI Handbook of Crime Scene Forensics.

Jim Fraser & Robin Williams, Handbook of Forensic Science (2009).

Hans Gross, Criminal Investigations (1962).

Max M. Houck & Jay A. Siegel, Fundamentals of Forensic Science (3d ed. 2015).

D. P. Lyle, Forensics for Dummies (2d ed. 2019).

Val McDermid, Forensics: What Bugs, Burns, Prints, DNA, and More Tell Us About Crime (2014).

(RAJ DISCLAIMER: I am not a forensic scientist, just a fan of procedurals who read or skimmed some books. Trust none of these notes.)