Fingerprints
Fingerprints
• Made of grooves and friction ridges.
• Fingerprint pattern is left by friction
ridges.
• Ridges evolved for efficiency – they are
useful in holding things, opening doors
etc.
• Are individual and not shared
• Remain unchanged throughout life
• show general patterns so can be
classified
• all people share these patterns.
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Classifying Prints
FBI has more than 200 million fingerprints on file.
Basis for fingerprint matching and identification Whorls
Loops
Arches
• Everyone has them.
• How they have them is unique.
• Each person has a different number of these types of
patterns and the pattern varies from fingertip to
fingertip on each person.
Arches
• 5% of all pattern types
• Ridgelines rise in center to create
wavelike pattern
• Can be plain or tented; tented rise more
sharply.
Loops
• 60% of patterns
• One or more ridges that double back on
themselves
• Can be radial where loops flow down
towards radius (thumb side)
• Or can be ulnar, where loops flow
towards ulna (little finger side)
Whorls
• 35% of patterns
• Look like whirlpools of ridgelines
• Can be plain – concentric circles like a bulls
eye
• Central pocket whorls look like a loop with a
whorl at its end
• Double loop whorls – two loops that collide
to produce S shape
• Accidental loop whorls are different from
other whorls – irregular.
The Henry System
• Individual prints are assigned scores
based on where whorls show up in the
10 fingerprint set; final discrimination
of prints is always manual.
AFIS – Automated Fingerprint
Identification System
• Computer scans and digitally encodes
fingerprints and stores that information.
• Computer can search thousands of files per
second, when a match is found agent has to
verify them individually.
Types of fingerprints
• Patent prints – occur when blood, ink paint, dirt or
grease on fingers are left behind as a visible print
• Plastic prints – have a 3-D quality and occur when
there is an impression in wax, putty, caulk, soap,
butter or dust
• Latent prints – invisible. They need lighting and
luminal to be seen.
• Finding Latent prints – can be found since
fingers pick up oils and dirt from skin.
• Latent prints can be powdered and lifted.
• Using Cyanoacrylate vapor (Superglue) print can then be photographed or treated
with a fluorescent dye that bind to print.
Other ways use iodine, ninhydrin and silver
nitrate
Trace Evidence
• any tiny piece of physical material that
can be transferred from one person to
another, or between a person and a
crime scene.
• Links suspects, places and objects.
• Examples = hair, human or pet, fiber
from clothing, paint chips, dust, sand,
pollen, seeds.
• HAIR
• Grows from cells in a follicle.
• Hair shaft - 3 parts –medulla, cortex
and cuticle.
• (Analogy = pencil; lead = medulla,
wood = cortex, cuticle = paint) Cross
section of human hair
• Cortex contains pigment - gives hair its
color.
• Medulla = central core.
Cross section
of human hair
• Cuticle covers shaft; looks like scales
(analogy = roofing tiles); 3 distinct
patterns
Coronal scales –
look like stack of
paper cups
Photomicrograph
to show scale
pattern on human
hair – imbricate
pattern
• Medullary index = width of the medulla
compared to the overall width of the hair.
• Most animals’ medullary index is 0.5 or
greater.
• In humans the index is about 0.3 (medulla
is about 1/3 the width of the hair)
Dog Hair
Horse Hair
Human Hair
• Dyed Hair
• Hair grows about ½ inch a month;
• Un-dyed hair root gives the criminalist
plenty of data: can tell if the hair has
been bleached, dyed, treated, and when
it was last done!
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When comparing the following
color and width
Pattern of the medulla
Color and pattern of pigments
Cuticle pattern
• If a hair matches a person in all 4 aspects
then there is a 1 in 4500 chance the hair
found at a crime scene came from a suspect.
• Animal hair cuticle has a different scale
pattern.
Human Hair
Deer Hair
Bat Hair
• Synthetic fibers have no medulla. Fibers
may appear twisted. A cross section of a
fiber may appear trilobal under the
microscope. Also, fibers may contain
delusterants (chemicals added to fiber to
get rid of shininess) that will appear as
little dots under the microscope.
Ballistics
Types of Bullets
• Lead bullets
• Used in low velocity weapons (small
handguns, and .22 or .25 caliber rifle)
• Deform and fragment when they hit the
target
• Cause severe tissue damage
• Lead alloy bullets
• Harder then lead bullets
• Deform and fragment less when they hit
the target
• Penetrate deeper
• Semi jacketed bullets• Thin layer of brass coats the sides with
soft lead nose exposed
• Used in low velocity and high velocity
weapons (.357 and .44 magnum
handguns and high powered rifles)
• Slightly hollow nose will deform and
fragment and cause a lot of tissue damage
when it hits the target
• Fully jacketed bullets• Completely covered with brass (full metal
jackets)
• Used in high velocity guns (.44 magnum
handguns and high powered rifles)
• Greatest penetrating power
Determining Caliber
• Caliber = measurement of the internal diameter of the gun
barrel
• Rifles and handguns measured in inches or mm (example = a
.38 caliber handgun barrel has an internal diameter of .38
inches)
• If a bullet is intact and not severely deformed its diameter can
be measured at the base to determine the caliber of the gun it
came from
Shell Casings
• Portion of the cartridge that remains after the powder
explodes and the bullet is gone. Retain several
important markings:
• Impression left by the firing pin
• Knowing where the firing pin struck (center or around the
edge) helps to narrow down the type of gun it came from
• Head stamps
• Manufacturer’s initials or log, caliber or gauge, and
cartridge types or stamped on shell’s when they’re made
• Extractor and Ejector Marks
• Made by automatic and semiautomatic weapons that are
unique to each weapon
Bullet Markings: Rifling and
Ridges
• Rifling = Spiral grooves etched in to the side
of gun barrels to make bullets spin as they
are shot (makes bullet more accurate)
• Results in Lands and Grooves (striations) on
the bullet
• Lands – High parts between rifling marks
(raised part on bullet, tends to be wider)
• Grooves- low part in rifling marks (section
that’s been gauged out by gun barrel)
contains more pronounced striations
• Bullets shot from the same gun will exhibit
the same striation pattern