Trace Evidence
Hair and Fiber Samples
Trace Evidence
• Trace evidence is circumstantial evidence
• It will not point to just one person
• It can show a certain person was in a certain
place.
• People have been found guilty just on
circumstantial evidence.
Forensic Examination of Hair
• Hair falls from the body every day
• Unless skin is attached to it, it is hard to use
DNA to say whose it is.
• Usually, hair is removed by force at a crime
scene.
Morphology of Hair
• Hair is made from skin cells
• Hair has 3 parts – the cuticle, the cortex, and
the medulla
1) Cuticle
• Hair resists decomposition
• Corpses have been dug up with full heads of
hair.
• It retains its structure for a long time.
Morphology of Hair
• Cuticle = a scale structure covering the outside
of the hair.
• Made of overlapping scales that point toward
the tip of the hair.
• The skin that makes of the cuticle are covered
in a protein called keratin.
• Finger and toe nails are also skin cells covered
with keratin.
Morphology of Hair
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3 basic patterns to cuticles
Coronal, spinous, and imbricate (page 413)
We use these when matching samples
Looking at the scale pattern tells us if it is
human or animal.
2) Cortex
• This is the main body of the shaft.
• Looks like blocks stacked end to end.
Morphology of Hair
• This is where the pigments for color are found.
• How the pigments are shaped and distributed
gives us points for comparison.
3) Medulla
• It is a collection of cells running up the center
of the hair.
• In animals, it makes up ½ the width of the hair.
• For humans, it is generally 1/3 or less.
Morphology of Hair
• This percent can change from hair to hair on
the same person.
• The medulla can be continuous
(uninterrupted), interrupted (with space
between), fragmented (no real pattern) or
absent.
• Humans usually have fragmented or absent.
• Human medullas are shaped like a cylinder.
Morphology of Hair
• A cat’s medulla looks like a string of pearls.
(page 416)
• There is a database of the 35 most commonly
encountered animals.
3) The root
• Root contains what is needed for hair to grow.
• The initial growth phase lasts 6 years
• When it is pulled from the root, it will have a
follicular tag
Morphology of Hair
• We use the tag for DNA samples
• Once the root stops growing, it shrinks.
• The hair will last 2-6 months before it falls out.
Identification and Comparison of Hair
1) It tells us if it is animal or human
2) Will it match the suspect or victim
• We will get samples from the scalp or pubic
regions for comparison.
Factors in Comparison of Hair
• Comparing human hairs has to be done
carefully
• We match color, length and diameter, does it
have a medulla, and the shape and color
intensity of the pigments.
• Dye can be seen in the cuticle
• It actually looks like another layer outside of
the cuticle.
Factors in Comparison of Hair
• Bleaching hair removes pigments and looks
yellow under the microscope.
• If the hair has grown, it is very easy to tell the
difference between colored and natural.
• Hair will also collect poison.
- This is useful in cause of death.
• We use a comparison microscope to look at
hair samples.
Factors in Comparison of Hair
• It is easy to tell the differences when they are
side by side.
• Because hairs vary on one person, you will
need several samples from different parts of
the body.
• Human error is the biggest problem is
matching samples.
• Once a match is made, it has to be confirmed
with DNA.
Factors in Comparison of Hair
• It is fairly easy to tell which part of the body
the hair came from.
- Use length, medulla type and uniformity.
• You can generally tell race from hair, but it is
not 100%.
• We cannot tell age from hair.
• We can use DNA to figure out the sex.
• If hair falls out, there is no root ball attached
to use DNA.
Individualizing Human Hair
• We can find nuclear DNA in the root or on a
follicular tab.
• The follicular tab is the best place to find
nuclear DNA.
• Getting hair roots for DNA is easier while the
hair is in the growth phase.
• Unfortunately, most hair at a crime scene is
hair that fell out, not pulled.
Individualizing Human Hair
• The exception is for violent crimes.
• If hairs have fallen out, we can use
mitochondrial DNA.
• This is DNA that is passed from the mother to
the child.
- Nuclear DNA has parts from both mother and
father.
• Mitochondrial DNA is easier to use because
there are more copies.
Individualizing Human Hair
• This means you can use a 1-2cm length to get
DNA.
• But if people are related, you cannot use this
type of DNA.
Collection and Preservation of
Hair Evidence
• If you have a suspect hair, you need
standard/reference hairs for comparison.
(Controls)
• Get these from the victim and other people at
the crime scene.
• You have to know what part of the body the
hair came from to get the right correct
reference hairs.
Collection and Preservation of
Hair Evidence
• Hair from a crime scene usually comes from
the head or pubic regions.
• For the head, you need 50 reference samples
• For the pubic regions, you need 25 reference
samples.
• You need the full length of hair for
comparison.
• Have to pull the hair or clip it at the scalp.
Forensic examination of Fibers
• Just like hair, fiber is left behind at a crime
scene.
• It is especially important in cases that involve
personal contact.
- Homicide, assault, rape
• Fibers can be left behind in a hit and run or in
screens or glass during a break in.
• Since we cannot connect fibers to one person.
It is circumstantial evidence.
Types of Fibers
• We used to use only natural fibers
• Starting in 1920, we started creating
manufactured fibers.
- rayon, nylon, spandex, polyester, etc.
• Fibers come from clothing, carpeting, drapes,
wigs, artificial turf, etc
Natural Fibers
• Made entirely from animal and plant sources
Types of Fibers
Animal fibers
• Wool (sheep), mohair, cashmere (goats), fur
fibers (mink, beaver, etc.) and others
• We use the same procedures as with human
hair when identifying animal fibers
• We still need reference samples to compare
with
Types of Fibers
Plant fibers
• The most common plant fiber is cotton
• Since white cotton is used so much, it is hard
to match to a person.
• The combination of colors in cotton makes it a
little easier.
Manufactured fibers
• Rayon was created in 1911 and Nylon in 1939
Types of Fibers
• Originally they were created by regenerated
fibers.
• These were made from raw materials made
from cotton or wood pulp
• This is treated chemically, dissolved, and
forced through holes in a spinning jet to make
fibers.
• Today they are made from synthetic fibers.
Types of Fibers
• Polymers = long chains of molecules put
together from smaller units.
• 1939 – nylon created by accident
• A glass rod was put in a beaker and pulled out
a strand that hardened when it hit air.
• Polymers are the basic chemical substance of
all synthetic fibers
• Polymers are also used to make plastic, paint,
adhesives and synthetic rubber.
Types of Fibers
• Matter is made of molecules
• Molecule = elements put together to make a
new substance
• Ex) H20, C6H12O6
• Most molecules have less than 100 atoms in
them.
• Polymers have millions of atoms per molecule
• These are called macromolecules
Types of Fibers
• Polymers are made of chains of monomers
• Since there are lots of different monomers,
the number of combinations is almost
unlimited
• Nature also make polymers
• Proteins for animal hairs are polymers
• DNA is a polymer
• Starch is a polymer – it is chains of sugars put
together.
Identification and Comparison
of Manufactured Fibers
• If you are lucky you can get a piece of torn
fabric that you can match with another piece
of fabric
• Normally, you only get a strand or two
• You look at it under a comparison microscope.
• Looking for color and diameter
• If these two things do not match, you stop
doing tests.
Identification and Comparison
of Manufactured Fibers
• Other things you look for are stripes within
the fiber or pitting in the fiber so you can
make it look shiny.
• Since each manufacturer has its own colors,
we can match a fiber to a specific
manufacturer.
• This can link a fiber to a house or car.
How We Analyze Fibers
1) Chemical composition
• We can find its general type (nylon)
• We can find a specific type (nylon 6)
• Fibers can be separated into 24 different
categories.
• People in the textile industry have lots of tests
to determine the class of the fiber
• Need lots of fabrics to do these tests.
How We Analyze Fibers
• Criminalists only get a fiber or two to work
with.
2) Bifringence
• Polarized light that passes through a fiber will
spilt in two.
• The fiber will bend the light (refraction)
• Each type of fabric refracts the light differently
• You can look at a single fiber without
damaging it.
How We Analyze Fibers
3) Infrared absorption
• Different fibers absorb different amounts of
infrared light.
• We can do this while looking at it under a
microscope.
• It will not damage the sample so other tests
can be run.
Significance of Fiber Evidence
• We cannot say that a single fiber came from a
single source
• Databases exist for carpeting, but that’s it.
• New fabrics come onto the clothing market all
the time, no database for clothing.
• With the huge amounts of colors and fiber
types, to make the evidence mean something
we have to do lots of test.
Significance of Fiber Evidence
• Color, size, shape, how it looks under a
microscope, chemical composition, dye type
and content.
• Unless it is denim or white cotton, we should
be able to match it.
• The more different fibers you can connect to
someone, the stronger the evidence is.
Collection and Preservation
of Fiber Evidence
• Because trace evidence has become so
important, crime scene investigators have to
do a better job of finding it.
• It occurs at almost every crime
• You really have to look for it.
• Clothing is put in to paper bags, each in a
separate bag.
• This is so you do not get cross contamination
Collection and Preservation
of Fiber Evidence
• Carpets, rugs and bedding are folded so no
fibers get out.
• Car seats are covered completely with plastic
• Knives are protected to preserve fiber
evidence
• Tape is used to pick fibers off of skin
• If fibers are loose, they are put in paper, the
paper is folded, and then put into another
container.
Collection and Preservation
of Fiber Evidence
• Once it gets to the lab, it is a long and tedious
process to get through all of the evidence.
• The crime scene investigator can make this
easier by only collecting pertinent trace
evidence.
• The better the investigator is, the fewer the
samples, the quicker the tests can be run.