Forensics Study: Impression Evidence, Firearms, and Fingerprints

Footwear, tire tracks, toolmarks

Main types of impression evidence.

Class characteristics

Size, shape, pattern (shared by many).

Accidental characteristics

Unique cuts, wear, flaws.

4 types of impression conclusions

No comparative value, Elimination, Class characteristics only, Positive individualization.

Crucial before collecting suspect shoes

Photograph, document size/model, ensure the suspect wore them.

2-D impressions

Length and width.

3-D impressions

Includes depth.

Best surfaces for electrostatic dust print lifters

Dry, hard surfaces with dust-based impressions.

Ideal packaging for impression evidence

Cardboard or rigid container, not plastic.

3 main methods for recovering impressions

Photography, lifts (gel/electrostatic), casting (3D).

Oblique lighting

Used to reveal depth and texture in impressions.

Common casting materials for snow

Snow print wax, sulfur, auto body primer.

Collecting firearms at a scene

Make safe, document, photograph, avoid metal tools, package in a box.

2 types of handguns

Pistols (semi-automatic), revolvers.

5 action types of long guns

Lever, bolt, hinged frame, pump, automatic.

Rifling

Spiral grooves inside the barrel; improves accuracy/stability.

Center-fire vs rim-fire primers

Center-fire: primer in center; Rim-fire: primer around edge of base.

Class characteristics in expended bullets

Caliber, # of lands/grooves, twist direction, impression widths.

Bullet wipe

Dark ring of residue left as bullet enters target.

Chemical tests for distance determination

Greiss (nitrites), Sodium Rhodizonate (lead), Dithioximide (copper).

Contact vs intermediate gunshot wounds

Contact: soot/scarring; Intermediate: stippling/burns; Close: tighter pattern.

IBIS system

Ballistics database comparing casing markings to known samples.

Henry Faulds

First proposed using fingerprints in criminal investigations.

Will West / William West case

Discredited anthropometry and proved fingerprint uniqueness.

3 main fingerprint pattern types

Loops (~60-65%), whorls (~30-35%), arches (~5%).

Visible, plastic, and latent prints

Visible: in blood/ink; Plastic: 3D in soft surfaces; Latent: invisible, need development.

Purpose of enhancement

Increase contrast of visible prints or make latent prints visible.

3 types of fingerprint powders

Traditional, magnetic, luminescent.

Super glue fuming

Used to fix/develop latent prints by reacting with residues.

Chemical for fingerprint development on paper

Ninhydrin (reacts with amino acids); DFO (for currency).

AFIS

Automated Fingerprint Identification System; compares ridge details across databases.

Preserving fingerprint evidence

Photograph, lift with tape (if needed), package in clean paper/cardboard.