CSI Facts

CSI [sē] [ĕs] [ī]

The scientific study of physical evidence at the scene of a crime.

What is CSI?

Crime scene investigation is a science known as forensics. For every crime, there is evidence left behind that can help authorities reconstruct the facts behind the incident. Skilled professionals analyze every bit of the scene until they know all the details of a crime including, sometimes, who did it. Let's take a look at this fascinating job and the science behind it.

looking at an evidence bag


The word forensic comes from the Latin word forensis meaning public. It's related to the word forum, which in ancient Rome was a market, a place of public discussion, or a court. So our modern term forensic, which we use to mean the use of scientific methods in solving crimes, has its roots in the Latin words for making something public or bringing something to court. Today, any science that is used in the field of law is a form of forensic science.

forensic case boxes


witness being questioned

Forensic scientists study lots of evidence at the scene. Their first line of fact-finding is to talk to anyone who was a witness or might have been a witness without realizing it. Witnesses can offer important clues. Sometimes the victim makes a good witness. But because people can forget or remember events incorrectly, physical evidence is usually more reliable.

Try this with a friend. Show your friend a photograph containing a number of people — say 3 or 4 — doing something in a location not familiar to your friend. Allow them to look at the picture for under a minute. Don't tell them why. Then ask them detailed questions about the picture. What color was the hair of the tallest person in the picture? What was the person wearing that was sitting? Which person had a hat? See if your friend caught all the details you are searching for. See why witnesses don't always get everything right. They didn't realize they were going to be a witness when the event was happening. Fear and other emotions can also affect a person's memory.


crime files and evidience

Some of the evidence that can exist at a crime scene:

  • Soil
  • Glass
  • Plants
  • Fibers
  • Fingerprints
  • Tire tracks
  • Splatter
  • Footprints
  • Liquids
  • Chemicals
  • Handwriting
  • Drugs
  • Bullets
  • Hair
  • Blood
  • DNA
  • Broken or bent objects

These are just a few of the things that a forensic scientist might find and study at a crime scene.

Cross Contamination

When studying a crime scene, it is extremely important that evidence doesn't get cross contaminated. This means that evidence doesn't get mixed with other evidence, or that it doesn't get touched or moved by people who are on the scene. To prevent cross contamination, crime scene tape is placed on the site to keep people from walking into or touching any part of the area being investigated.

collecting evidence

Investigators take other precautions too, like wearing gloves so their own fingerprints don't end up on the evidence. Each piece of evidence that they collect is stored in a separate plastic container before taking it to the crime lab to study further.

Other Challenges

forensic equipment

Outdoor crime scenes present some difficult challenges for investigators. Animals or other humans can accidently cross through a crime scene and destroy evidence before the investigators get word that a crime has been committed, especially in public outdoor areas like parks.

Time is crucial — having to investigate an outdoor crime scene at night when it is dark may cause forensic experts to overlook important evidence, but waiting until morning light allows weather conditions like rain or temperature changes to alter evidence.

When is it used?

collected evidence to be examined

TV programs would have most people thinking that crime scene investigators are hunting for clues in a murder or a kidnapping. That is often done, but forensic scientists can also solve cases involving burglary, break-in, carjacking, poaching, bank robbery, fraud, forgery, a traffic accident, and many other crimes.

Forensic scientists have even been called on to identify dinosaur bones, crimes committed centuries ago or to determine the cause of death for historical celebrities such as ancient kings and pharaohs.



DNA is often used to identify victims or suspects or to rule out people involved in criminal activities. But what is DNA and how can it be used in such instances?

DNA is like a secret code that exists in all of our cells. DNA is a microscopic instruction plan that holds details about us such as what we look like, what hand we write with, and even how tall we will grow to be. Everyone and every living thing has DNA. No two people have the same DNA unless they are identical twins. Because every living thing's DNA is different, it is used by investigators to determine the identifying details of a person, from hair color to gender. Scientists are currently working on discovering all of the secrets that DNA holds. Learn more about how DNA is used in forensic work.

In forensics, scientists can take a sampling from the inside of someone's cheek or their blood and compare the DNA of one sample to the DNA of another sample to be able to tell if they came from the same person. This can be very helpful in proving that a suspect is actually NOT guilty. This method of crime-solving has been used regularly since 1992.


suspect being fingerprinted

Fingerprints are the small ridges on the tips of our fingers and are different on every person. When a person touches a table or another surface, they leave behind a telltale sign of their fingerprint. Forensic investigators can remove this fingerprint using a special powder and some tape. They then compare this evidence with a fingerprint that has already been collected from the person to whom they are planning to compare.

Fingerprint specialists evaluate the small curves and whorls of the fingerprints to determine if they are the same. There are a series of fingerprint patterns which are similar in all people. Take a look at these patterns to learn more about fingerprinting.

fingerprint evidence

Wildlife Forensics

collected evidence

Crime in the animal world happens more than most of us know. Forensic investigators use the same methods and tools as those who solve crimes against people. They can find out how an animal died, how long ago it happened, and track down if it was killed by another animal or by a human. By teeth marks and hair left at the scene, they can even figure out what kind of animal did the killing. Sometimes it is important to find out if a wolf or a bear is killing farm animals or is in an area frequently visited by hikers or campers. This is a safety issue for all of us who love the outdoors.

Animals are sometimes hunted illegally. This is known as poaching. A forensic scientist can use evidence left at the scene of a dead animal to help authorities arrest the person or the people who killed an endangered animal, hunted more than they were permitted, or who was hunting outside of the rules.

Want To Be a Forensic Scientist?

scientist looking at evidence

Here are just a few of the possible jobs available in the world of forensic science:

  • Medical examiner — performs autopsies
  • Chemist — analyzes chemicals found
  • Forensic engineer — can determine how a structure collapsed
  • Toxicologist — tests for poisons
  • DNA analyst — uses DNA to identify victims or suspects
  • Lab technician — studies blood and other body samples
  • Computer technician — records digital information for safe keeping
  • Fingerprint expert — identifies specific patterns in fingerprints
  • Handwriting expert — determines handwriting patterns
  • Psychiatrist — studies crimes and the people involved to help prevent future crimes
  • Anthropologist — studies bones to determine age, gender, race, etc.
  • Ballistics expert — determines the gun and ammunition used in a crime

Lots of other professionals can be called on in particular situations — plumbers, electricians, nurses, doctors, dentists, and anyone else who might have information specific to the details of a case.

Top 10 Questions

November 2010

Thanks to Matthew Gamette, Forensic Services Quality Manager, and Natasha Wheatley, Latent fingerprint and shoe print/tire track examiner, Idaho State Police for the answers.

  1. How do you find fingerprints?

    We find fingerprints by using different chemicals or powders to make them appear. Even though the powder method (dusting) is more well-known, we actually develop more prints with chemicals. (From Kate in Mrs. Schultz's class at Dalton Elementary School in Dalton Gardens)

  2. When you find evidence, like a fingerprint, how do you know who it belongs to?

    Fingerprints are very unique to people. The ridges on the surface of a person's skin develop when they are in the uterus. The pattern of these ridges is what makes each person's fingerprints unique. There are three types of patterns: arches, loops, and whirls. We use these patterns to determine who the fingerprint belongs to. (From Dylan in Mrs. Hudson's class at Dalton Elementary School in Dalton Gardens)

  3. If you found a fingerprint on a wall while investigating a crime, how would you know if it belonged to the criminal or the owner of the house?

    When we investigate a crime, we also collect elimination prints. These are prints from anyone who could have touched the wall in the house. We get fingerprints from the owner of the house, compare them to the print on the wall, and then eliminate the prints of those who aren't suspects. (From Tallon in Mrs. Hudson's class at Dalton Elementary School in Dalton Gardens)

  4. Is it harder to find clues in bad weather like snow?

    It can be really hard to find evidence that is buried under the snow. However, sometimes the snow makes it easier because the evidence may be on top of the snow. For example, a suspect walking across the snow may leave a nice set of footprints, or a car driving on a blanket of snow may leave good tire track evidence. (From Kate in Mrs. Hudson's class at Dalton Elementary School in Dalton Gardens)

  5. Is it possible to find out if a thief is a boy or a girl?

    That is possible. In fact, that is what DNA science is all about. There are specific areas on your DNA that make you male or female. So, when we run a DNA test, we look to see if the suspect is male or female. This is a very important part of an investigation. (From Emma in Mrs. Hudson's class at Dalton Elementary School in Dalton Gardens)

  6. How many different types of DNA are there?

    DNA is made up of several bases, or chemicals. These bases are organized in a number of different ways so every person's DNA is unique and has a different DNA profile. We look for these differences when we are trying to solve a crime. (From Lauryn in Mrs. Woodall's class at Dalton Elementary School in Dalton Gardens)

  7. Do twins have the same fingerprints?

    They do not. When the twins are developing before birth, there are different pressures going on. They call this differential growth. Even identical twins do not have the same fingerprints. (From Molly, Carson, Taylor and Alec in Mrs. Peterson's class at Owyhee Elementary)

  8. Do you use special cameras in your work?

    We do. They are not the typical point and shoot cameras that we usually see. They are very high end, expensive cameras that sometimes come with special filters to let us view blood on very dark clothing. (From Troy in Mrs. Schultz's class at Dalton Elementary School in Dalton Gardens)

  9. How many different types of DNA samples can detectives analyze from a crime scene? What are you looking for?

    We are looking for anything that comes from a body. DNA is in almost any cell in our bodies. Whether we look at a person's skin, hair or blood, a person's DNA is the same in all of those different types of samples. (From Kaitlin in Mrs. Woodall's class at Dalton Elementary School in Dalton Gardens)

  10. What is the hardest evidence to find at a crime scene?

    The hardest evidence to find would probably be trace evidence. We're talking about hairs, fibers, and things like that from material. Also, things that you can't see are hard to find, like DNA. (From Porter from Trail Wind Elementary School in Boise)