Crime scene investigation has developed since Sherlock Holmes’ day, although careful examination of evidence is just as important today! The science that deals with crime investigation, or science that is applied to law cases, is called forensic science.

Modern techniques for solving forensic cases started to develop in the late 19th century. In the 1890s Francis Dalton summed up earlier studies in a book about fingerprints, demonstrating that each person has unique prints that do not change with age. At the same time, Argentina became the first country to use fingerprinting in criminal cases. In 1904 the New York City police force began collecting the fingerprints of prisoners. Then the first crime lab, using scientific techniques to examine evidence, opened in France in 1910. Since then, equipment has improved with technology, incorporating computers and other high-tech instruments.

Almost every scientific field could be involved in forensics in some way. Here are just a few of the professionals who might work on a case: chemists, psychologists, pathologists (disease), botanists (plants), odontologists (teeth), entomologists (insects), and anthropologists (humans). So if you study forensics, you might use physics for comparing densities and investigating bullet impact; chemistry for identifying unknown substances, such as white powder that could be a drug; biology for identifying blood and DNA; and earth science for soil evidence.

The foundational principle of forensics is that a person always brings something to the crime scene and always leaves something. Often this is in the form of trace evidence: hair, fiber from clothing, chips of paint or glass, residue, and other “traces” of who was there. Hair evidence is examined in a lab under a compound microscope. Experts look at the color and shape of hair, but they also find out what stage of growth the hair was at in order to determine how it was left behind. At the last stage, it might have been shed naturally. But if the hair was at an early stage, it might have been torn out by hard physical contact. Think of situations where different kinds of hair – not just human – might be good clues.

In cases of violent or suspicious deaths, a medical examiner looks at the victim. The examiner uses body temperature, the presence of bacteria, how digested the victim’s food is, and other means to find out when the person died. He also looks for wounds or marks that suggest a struggle, and does an autopsy to examine the inside of the body as well. In cases where a death might be murder made to look like suicide, an investigator uses signs in the victim’s body. In a drowning death, are there diatoms (microscopic water creatures) inside the body, from swallowing water? If so, do they match the diatoms found in the water where the body was? In the case of a burning, is there carbon monoxide, from smoke inhalation, in the bloodstream? If there is not, this would suggest that the person was already dead before the fire.

Identification is essential for getting the right suspect, but each victim has to be identified, too. In both cases, fingerprints, birthmarks, scars, tattoos, bones, and teeth can be used. Although fingerprinting is often useful for nailing down a suspect, not everyone has had his or her fingerprints recorded, and the police might not have access to someone’s prints. One method that is more useful than fingerprinting is dental imprints. Teeth marks made by a criminal or teeth from a dead body can be used for accurate identification, even if only partial evidence is left. The teeth or bite marks are compared to dental records, especially X-rays. After the 1995 Oklahoma City bombing, dentists were able to identify about 25% of the victims based on teeth recovered from the site.

Science Project: Forensic Science Experiments

Study a Crime Scene of Your Own

See how many clues you can identify in your own “crime scene.” Choose a room (e.g., kitchen, living room, bedroom) or part of a room and go over it carefully, finding any trace evidence such as hair, clothing fibers, and chips of paint. You can collect these with a pair of tweezers and place them in envelopes or Ziplock bags to identify later. Are there any prints or scuff marks on the floor from shoes? Bits of soil or rock that might have been tracked in? To be thorough, record all of these clues and make sketches in an investigation notebook.

If you have a microscope, compare different kinds of hair at high power magnification. (You can also use a 10x or stronger magnifying glass.) Examine different cloth fibers, too – try cotton, wool, and rayon or acetate. Make a wet mount of the hair or fibers by putting a drop of water on a microscope slide, adding the specimen, and pressing a cover slip down on top. What does each specimen look like? Is it smooth or rough? How do the ends look? Compare miscellaneous hair and fibers you pick up from the carpet or couch. Can you tell what kinds of fibers they are? Where did they likely come from?

Check out any dental evidence in your crime scene. Then, if you have some willing suspects, make impressions of their bites and compare the impressions to the evidence you found. A simple way to make impressions is to carefully bite down into an apple or other soft food, but you can also bite into a folded sheet of white paper with a piece of carbon paper inside. Be sure to get both back and front teeth in the bite impression.

Prints All Over the Place

Collecting fingerprints is not that hard to do at home, and we don’t mean with the ink and stamp method! Some fingerprints are visible – you can see marks left on a surface by dirty or oily fingers. Dusting is usually used for this type. Other prints are latent – you can’t see them, but there are marks left by sweat and other organic residue from fingers. Fuming is often used for these.

If you have a magnifying glass, inspect your fingers. The unique patterns on your fingertips are caused by ridges in the dermis, the bottom layer of your skin. These patterns are fully developed in human beings just seven months after conception, while the fetus is still in the womb. The three typical patterns are loops, whorls, and arches. (Look at examples of different fingerprint patterns.) Your fingerprints are different than anyone else’s, but did you know that fingerprint patterns tend to run in the family? If your fingerprints are a whorled pattern, one of your parents probably has a whorled pattern, too. It’s just not exactly like yours!

To dust for fingerprints, sprinkle talcum powder or cornstarch on dark surfaces and cocoa powder on light surfaces (like the outside of a drinking glass) where there are visible prints. You can use a small paint or makeup brush with very soft bristles to gently swipe off the excess powder and leave the print. Use clear tape, sticky side down, to lift the print and then stick it to an opposite-colored paper. What kinds of patterns do you see?

Another method for collecting fingerprints is called fuming. Certain chemical fumes react with the sweat and other organic residue left in latent fingerprints. You can experiment with this yourself: all you need is an aluminum pie plate or square of aluminum foil folded in fourths, a glass jar, superglue, and a smooth object like a pen or a marker lid. Wipe down the object, then hold it for a minute so that your fingers leave latent prints. Set the object inside the jar. Next, put several drops of superglue on the middle of the pie plate and turn the jar upside down over it. The strong chemical fumes from the cyanoacrylate in the glue will react with the residue from your fingers. You should see white fingerprint images on the object after a half hour or so. Professionals also use ninhydrin (which reacts with amino acids in latent prints) and silver nitrate powder developed under a UV light.

Fingerprint everyone in your house. What patterns are most common? Based on your latent and visible fingerprint collection, which surfaces “reveal” prints best? (To make these fingerprint records, it will be easiest to use ink or marker rubbed onto the fingertips and then stamped onto a white paper or card.)

Who Wrote It?

Chromatography is used to identify different inks. Say someone committed a crime by changing the dollar amount on a check. Using chromatography, an investigator could tell whether more than one ink pen was used to write on the check and whether the suspect’s ink pen could have been used. How does it work? Well, ink is not really made up of one color: there are actually different pigments making up one ink. In chromatography, the ink is soaked in a solution so that the different pigments will “bleed” apart and the true colors be revealed. (As you might guess, there is a drawback: the evidence is destroyed in the process.)

You can see how chromatography works by doing this experiment. Fill a tall glass half way with water. Cut 3-4 strips of filter paper or of a heavy paper towel and attach the ends to a stiff piece of wire or a stick that can rest over the top of the glass. Next, make a large dot of ink about 1/2 an inch from the bottom of the strips, using a different brand of black marker, felt-tip pen, or ink pen for each strip. Set the strips in the glass so that the ends touch the water but the ink dots are above the water level. As the water soaks up into the paper, the ink will begin to separate into different colors. Note that some inks are not water-soluble; if the ink does not bleed, try using either nail polish remover or rubbing alcohol (stronger solvents that can dissolve the bonds in the ink) instead of water.

You can also look at “suspect” paper itself – are there watermarks or imprints from writing on top? Professionals also study handwriting and can analyze a sample of disguised writing to see if it has characteristics that match a suspect’s normal writing.

Flashback in History: Forensic Science & Archaeology

Forensic scientists don’t just work at crime investigations. They play a key role in understanding anthropological and archaeological finds, such as the so-called ice people and bog people – ancient bodies found either frozen or preserved in bogs. Both bogs and ice prevent bacteria growth that would rot bodies. Bogs also contains acids that tan a body’s skin like leather. Once these naturally-mummified bodies are found, scientists examine them for clues to their past. Did they die violent deaths? What did they eat? What did they do for a living? Forensics can help answer these questions, although sometimes the evidence is not enough to make conclusive judgments.

Studying the bones of a person can tell a lot about them. The shape of the pelvis and overall size of the skeleton indicates whether the body belonged to a man or a woman – women have a broader pelvis and men tend to have heavier bones. Forensic anthropologists who specialize in bones can often determine cause of death and even tell by scar tissue whether bones had been broken and healed.

Other scientists who solve archeological mysteries include botanists and pathologists. Botanists can identify trace amounts of fiber or pollen on clothing or even in a stomach’s contents, and pathologists can tell if a body suffered disease and whether it was killed or died naturally.

Science Links

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