We all love a good detective story. If
we can figure out "whodunit"
before the detectives on TV, we feel as smart as Sherlock Holmes! In
reality, solving a crime usually takes an awful lot longer than a
60-minute episode of your favorite police drama. Vast amounts of
evidence often have to be collected and processed, huge numbers of
witnesses may have to be interviewed, and it can take years (sometimes
even decades) before the person responsible for a crime is brought to
justice. Sherlock Holmes, the famous fictional detective from London's
Baker Street, relied on his powers of observation and deduction to
solve crimes that baffled the police. But in the real world, it's often
forensic scientists, working diligently out of the spotlight, who
provide the crucial pieces of evidence. How can you use science to
solve a crime? Let's take a closer look!
Photo: A forensic scientist has to find evidence that may be invisible to
the naked eye using sophisticated laboratory equipment and ingenious chemical tests.
Photo by Dean Calma courtesy of IAEA Imagebank published on
Wikimedia Commons under a
Creative Commons (CC BY-SA 2.0)
The place where crimes like theft, murder, and terrorist attacks
take place is called the crime scene. Once a
serious crime has been reported, detectives head straight for the scene and seal it off
to prevent anyone from removing or destroying any evidence that may be
there. Crime scenes can vary greatly in size. If a car has been broken
into in the street, the scene is effectively just the vehicle itself.
If terrorist bombs blow airplanes down from the sky, the crime scene can
extend over many miles on the ground as accident investigators struggle
to recover important bits of the wreckage.
A detective's first job is to try to make sense of the crime scene
and understand exactly what has happened. This isn't always as easy as
it seems. Imagine being called to a house by a neighbor who has heard
loud piercing screams. You discover smashed windows, scattered clothes
and papers, and a body shot dead in a bedroom. There's a shaken man
outside who's trying to tell you about a car he saw screeching away
from the house minutes after shots were fired. Are you investigating a
robbery? A murder? Maybe the dead person committed suicide and the
person who sped from the scene was racing off to get help? Perhaps the
witness was mistaken and the speeding car had nothing to do with the
crime? Has the person in the car gone on to commit other crimes
elsewhere? Maybe the man you're talking to is actually the murderer?
There are often many possible explanations for what has
happened. Establishing the exact sequence of events
immediately before and after the crime was committed is a vital part of any
Photo: A forensic scientist searches the wreckage of an aircraft that crashed over Greenland in
1962. Photo by Jeffrey Lehrberg courtesy of US Navy and Wikimedia Commons.
Ordinary detectives (police officers who investigate complex crimes)
are soon joined at the scene by specialist forensic scientists. These
are the people you see on the television
news, wandering round in the
background of crime scenes in strange white suits. The scientists wear
these suits (made from a disposable, papery-plastic protective material called
on top of their own clothes to stop them from contaminating the scene.
Their job is to find every single piece of evidence, no matter how
small, and take samples away for further investigation.
Picture: Every piece of evidence at a crime scene is meticulously
recorded. Photo by Kevin Stabinsky (USAG Fort McPherson) courtesy of
Depending on the type of crime and how it was committed, there can
be many different types of evidence at the scene. There might be
fingerprints on doors or windows, on drinking glasses left on tables,
or on many other objects around the scene. If someone has been shot,
there could be discarded bullets or powder
residues. In a sex crime,
there may be samples of blood, semen, hair, or discarded clothes. One
of the hardest things for a forensic scientist is separating out the
really important evidence from all the other things they may find. If
they discover hairs on a carpet in a bank where a robbery has taken
place, they could well be hairs from the robber's head. Equally,
though, they could be hairs from the hundreds or thousands of people
who passed innocently through the bank in the weeks or months before.
Somehow the forensic scientists have to sort out the one or two pieces
of really important evidence from the hundreds or thousands of bits of
irrelevant material they are certain to find.
Crime scenes cannot be preserved intact indefinitely, especially
when crimes happen in public places that need to reopen as quickly as
possible. So, apart from gathering evidence, detectives and forensic
scientists aim to preserve an impression of the scene using
photographs, sketches, and video recordings. The position of every
piece of evidence is carefully measured and recorded before anything is
removed. Items of evidence are stored in separate plastic bags and
A crime scene is the place where a crime is
discovered—but it's not necessarily the place
where the crime was committed. When people murder, they often
kill in one place but dispose of the body in an entirely different
location in an attempt to conceal what they've done. Serial
(murderers who carry out a series of attacks) may kill and dispose of
people in many different locations. In this case, the crime scene (the
place where a body is discovered) typically contains less useful
evidence, and the body recovered from the scene is where the forensic
scientists focus their search for clues.
Once the scene has been fully investigated, the body is removed to a
mortuary where a forensic pathologist will
carry out an autopsy (a word that originally meant "seeing with your own eyes"),
which is also called a post-mortem ("after death"): a detailed scientific
inspection of the dead person's remains to establish the exact cause of death. Pathologists
carefully examine the surface of the body for wounds and bruises, but
they also cut it open and weigh and inspect all the internal organs
looking for the tiniest of clues. They will look for obvious signs of
physical damage (such as stab wounds or signs of strangling), but
they'll also do tests to find chemical substances
that may indicate poisoning. Murderers are often clever and cunning and
they may try to disguise the real cause of death by mutilating the
body. For example, if a murderer has killed someone by strangling them,
he may attempt to burn the body to prevent the cause of death being
established. Or he may cut off the victim's hands to reduce the
likelihood of their being identified from fingerprints. Pathologists
effectively have to carry out a mini-investigation within the wider
criminal investigation to establish exactly how, when, and where
someone died. Where a body has suffered multiple injuries or wounds,
establishing which one caused the person's death is very important.
The human body is made from soft tissues built on a strong skeleton
of bones. The tissues disintegrate quite quickly after death, which can
make identifying a victim very difficult. Often detectives have to use
dental records and DNA (the person's
genetic material) to confirm who a person is.
Witnesses and suspects
Some of the most important evidence in a criminal investigation
comes from the things people see and hear. Police always appeal for witnesses
(people who have observed events before and after crimes take place),
both to help them establish what has happened and to find out who was
responsible. In complex cases, detectives can find themselves
interviewing hundreds or even thousands of witnesses. All these statements
(the written descriptions of what people say they saw or heard) have to
be checked and compared. To complicate matters further, statements
often disagree because people don't always remember events
accurately—especially in stressful situations or
if a long time has elapsed between the crime being committed and the
police asking questions.
Picture: Detectives search for witnesses at a crime scene.
Photo by courtesy of US Army.
Some of those the police interview may be suspects
thought to be involved in the crime) and they do not necessarily tell
the truth when they are questioned. Suspects are usually interviewed
much more thoroughly and formally than ordinary witnesses. These
interviews may be carried out at a police station with lawyers present,
and they are typically tape-recorded so anything the witness says can
be used as accurate evidence in court. Sometimes suspects are
interviewed repeatedly over a period of weeks or months to see if their
stories remain consistent. If suspects are held in custody (locked up
in a police station or prison), they are usually kept apart to prevent
them fabricating stories. They are also interviewed separately and
quizzed about any differences between their accounts of what happened.
Police sometimes use psychological tactics to make suspects admit to
committing crimes, but they are not allowed to use torture or other
violent methods (sometimes known as the third-degree).
You may have seen pairs of TV detectives using the
famous "good-cop, bad-cop" routine, where one tries to befriend a
suspect or act sympathetically, while the other is usually more angry
and accusing. Psychological techniques like these put criminals under
pressure and often yield impressive results.
Meanwhile, back at the lab...
While detectives are interviewing suspects and witnesses and trying
to understand the crime more clearly, the forensic scientists will be
examining evidence collected from the crime scene for clues. This sort
of evidence can be used in two ways. One hope is that the scientists
will discover something unusual among the evidence that sheds light on
who committed the crime. Suppose samples taken from a murder scene
include traces of a rare chemical discovered on a rug near the body.
Perhaps it's a chemical the murderer trod in on his shoes that he uses
in the course of his work? Identifying the chemical and where it came
from then becomes a crucial part of finding the killer.
Photo: A US army fingerprint specialist examines
fingerprints taken from counterfeit currency to find out
who they belong to. Photo by Colby T. Hauser courtesy of
U.S. Army Criminal Investigation Command.
If the police have a suspect, forensic evidence can be used to try
to link that person to the crime scene. Suppose, in the course of
interviewing witnesses and suspects, the detectives find someone whom
they think carried out a killing. If they can find something that
uniquely ties the suspect to the crime scene, such as a sample of the
suspect's blood or hair on the
victim's clothes, that could help to establish the
suspect's guilt. So, an important part of interviewing the suspect
involves taking fingerprints plus blood and DNA samples and trying to
match those with samples taken from the scene.
This kind of forensic investigation takes place at a police
laboratory packed with pieces of state-of-the-art scientific equipment.
It's here that forensic scientists carry out meticulous experiments on
evidence recovered from the scene (and samples taken from suspects) in
an attempt to shed more light on the crime. Just like chemists and
biologists in an ordinary science lab, forensic scientists use
ordinary optical microscopes for examining samples, but they also have powerful electron
microscopes for looking at evidence in much more detail. They can
use complex chemical techniques called mass
spectrometry and gas
chromatography to identify the precise chemicals contained in
unknown samples recovered from the scene. With DNA
they can attempt to match genetic information taken from a body or a
crime scene with samples taken from a suspect.
How good a detective are you?
Be your own detective! Here are four different ways of collecting
and examining evidence you can try at home.
Hunt for clues
Pick a room in your home that's used by several different members of
your family. See if you can find as much evidence as possible that
different people have been there. Look out for hairs, sweet papers, mud
from shoes, cast-aside toys, items of clothing, discarded crossword
puzzles, and any other important clues you can find. Seal off the scene
and don't let anyone into the room while
you're working. You might want to use a magnifying
glass and tweezers to recover the evidence you find. Using a pen and
paper, list each item of evidence you find and record exactly where you
found it. Store the evidence carefully! When you have enough evidence,
try to figure out who has been at the "scene". Later, you can confront
your "suspects" with the evidence and see what they have to say!
Photo: A magnifying glass is handy for working with tiny bits of evidence!
Take your fingerprints
It's easy to take your own fingerprints. All you need is an ink pad
(the kind you can get from a stationery shop). Roll one of your fingers
carefully onto the ink surface then press it carefully onto a sheet of
clean paper. Remember that you're trying to show up the pattern of
lines on the surface of your finger, so don't use too much ink. You
could try taking prints from your family and friends and see how they
compare with your own.
Picture: A typical fingerprint with an "arch" pattern. If you want to show up
your own fingerprints, run a pencil very lightly over your fingertip. If you still can't see the
pattern clearly, try taking a closeup photo and filter it to make the ridges show up, as I've done here.
If you don't have an ink pad, you can make
fingerprints using a thick, soft pencil. Scribble a thick layer of
pencil onto a piece of paper by going over and over the same spot. Then
rub your finger into the pencil "pad" so your finger is black. Now
press your finger onto a clean sheet of paper.
Be careful not to get inky fingerprints on your clothes, the
furniture, or other parts of your home while
you're doing this. Your parents will enjoy using
the fingerprint evidence against you if you mess up your home!
Ask a friend or family member to scribble a secret message on a pad
of paper. What they have to do is write something on the top layer of
paper, pressing down very hard with a ballpoint pen, then throw that
page away. Now take the rest of the pad and see if you can figure out
what they've written. Using a light pencil at a
shallow angle, scribble carefully over the indentations in the pad and
see if you can make the message show up.
Photo: Left: Write a secret message on the top
sheet (press really hard). Right: Now discard the top sheet and see if
you can show up the writing on the sheet underneath by scribbling at a shallow angle with a
How good a witness are you? Find yourself a calendar. Now pick a few
precise dates in the past—maybe one day ago, one
week ago, one month ago, three months ago, and one year ago. See if you
can remember what you were doing on each one of these dates. A day ago
and a week ago should be easy, but you might struggle with the older
dates. Do you think you'd make a good witness in a
Why do we need forensic science?
Why do detectives go to such enormous lengths to investigate? There
are two answers. First, justice is very important in our society and
it's vital that people who commit crimes should be
found and punished. Victims of crime need to know that the perpetrators
will not walk free; we can stop crimes from happening in future only if
we catch and punish the people responsible for crimes that have happened in the
Photo: "Equal Justice Under Law"—the words chiseled into stone
above the US Supreme Court in Washington, DC. Forensic science plays an important part in
ensuring that justice is done. Credit: Photographs in the Carol M. Highsmith Archive,
Library of Congress, Prints and Photographs Division.
Where serious crimes like murder
have been committed, proving that a suspect is innocent or guilty could
mean the difference between them walking free or being executed. With
stakes so high, suspects in a crime will go to great lengths to prove
their own innocence, and their legal teams will do their best to reject
dubious evidence, however scientific it may seem. Sometimes courtroom
cases turn into battles between opposing scientists who become
expert witnesses for the defense or the prosecution. A guilty suspect
could walk free if their legal team can refute the scientific evidence
presented against them. But an innocent person could be imprisoned or
executed if a jury accepts flawed evidence and convicts them. That's
why science matters so much in criminal investigations.
Discover Forensic Science by Lindsey Carmichael. Lerner, 2018. Billed for ages 8–12, grades 4–6, though I would put it toward the younger end of that range.
Crime Scene Detective by Carey Scott. Dorling Kindersley, 2007. Can you find the clues and solve the crime? Activities for younger readers.
Criminal Investigation (Science Fact Files) by Chris Woodford. Hodder Wayland, 2006. My own book about forensic science takes you on a trip from the crime scene to the court-room, via all the stages of investigation in between.
A Brief History of Scotland Yard by Jess Blumberg. The Smithsonian Magazine, September 27, 2007. Looks at the history of London's famous police force and some of its more dramatic cases.
A Hit in School, Maggots and All by Natalie Angier. The New York Times, May 11, 2009. Teachers find forensics is the perfect way to teach a broad range of scientific topics.
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