Trace Evidence

Fiber evidence is found in a variety of cases. When a struggle occurs between two people, fibers may be transferred from the suspect's clothes to the victim's clothes and vice versa. Carpet fibers from a home may adhere to a breaking and entering (B&E) suspect's shoes. A pedestrian struck by a vehicle may leave fibers from their clothing on the suspect's vehicle bumper or windshield. These are just a few of the types of crimes that may be solved by fiber analyses.

A questioned (Q) and a known (K) fiber can be compared using polarized light microscopy (PLM). The fiber type (i.e. Nylon, rayon, cotton, polyester, etc.), cross sectional shape, sign of elongation, and refractive index can be determined with PLM. Analytical instrumentation like FT-IR and Microspectrophotometry can be used to further identify and compare the Q and K fiber's chemical composition and color to determine if the two fibers could have originated from the same source.

Paint can be transferred from one vehicle to another in an accident. An automotive paint chip left at the crime scene or on a hit-and-run victim's clothing can be used to determine the make and model of the vehicle it came from. Paint transferred from a window to a B&E suspect's tool can place that tool at the crime scene.

Paint is examined with microscopy and several analytical instruments to determine its layer sequence, binder type, and pigment content. If the Q and K paints are found to be similar in all these analyses, then they could have originated from the same source.

MSP Laboratories examine powders and exploded/unexploded devices to determine what type of explosive may have been used. After the MSP Bomb Squad renders a device safe, they submit a sample of the explosive (when the device is unexploded) or the debris (after an explosion occurs) to the Trace Unit.

These items are then analyzed with chemical spot tests and analytical instrumentation (FTIR, SEM/EDX, and IC) to determine their chemical make-up. This information will then be used to inform the submitting agency what type of explosive was used. These results can then be compared to any evidence found in the suspect's possession.

Fragments of glass can be compared to determine if they originated from the same source. When a pedestrian is struck by a vehicle, fragments of glass from the headlamp area or windshield may transfer onto the victim's hair and clothing. A criminal unlawfully entering a building or vehicle by breaking through glass will likely get fragments of glass on their clothing and on the tool used to break the glass.

In both of the above situations, glass particles (questioned glass) found by examining the clothing can be compared to particles collected from the crime scene (known glass) to determine if they have a common origin. Properties of the glass like tint, thickness, UV fluorescence, density, and refractive index, must all be similar for the questioned and known samples to have originated from the same source.

Imagine you are carefully pulling out of your driveway late at night and your car is struck by a vehicle with its lights off. The police arrive and the person who hit you claims you pulled out in front of him. You explain to the police officer that the person who hit you did not have their lights on, so you did not see them. The other party replies that his lights were "definitely on". How is the officer to know who is right and who is wrong? The headlamp(s) from the vehicle in question can be submitted to the Trace Evidence Unit for examination. The filaments in the light bulbs are examined for oxidation, hot stretch, cold breaks, rainbowing, and fused glass particles. The analyst can then make the determination whether the headlamp(s) was on or off when the collision occurred.

Footwear impressions and tire impressions are examined in a similar fashion to each other. Impressions can be three-dimensional when left in snow or soft soil, or they can be two-dimensional when a dirty, bloody, or wet origin impression is left on a surface. Footwear impressions can be as individual as a fingerprint. There may be thousands of other people that have the exact same shoe as you, yet as you wear your shoe, the tread wears down uniquely to your walking style. Also, accidental scratches, nicks, and cuts are left on the bottom of your shoe, which may be unique. Tires undergo the exact same changes making them unique as well. 

Questioned impressions from crime scenes can be photographed, lifted, or cast with dental stone to compare to suspect shoes or tires. A comparison of tread designs or wear patterns can lead to a positive identification. An unknown footwear or tire impression can also be searched in laboratory databases to determine the possible make and model of shoe or tire that could have made that impression.

What is a fracture match? It is a comparison between two fractured, cut, broken, or torn objects to determine if they were at one time part of the same object. For example, if a car strikes a pedestrian and part of the grill breaks off and is left at the scene, it can be collected as evidence. Once the suspect car is found, the questioned piece of grill from the scene can be physically aligned to the car's grill to see if they were at one time one piece.

When an object is broken or torn, two unique edges can be formed. These edges can be compared visually and under a microscope to determine if their fractured edges re-align. If the edges fit together like a lock and key, they are said to form a fracture match to one another and the two objects were at one time a single object.

When a suspicious fire occurs, a fire investigator investigates the scene. If they suspect an ignitable liquid may have been used, the investigator can collect areas where they believe the accelerant was used. These samples are placed in nylon bags or sterile metal cans, sealed air tight, and submitted to the Trace Evidence Unit for examination.

The samples are then documented and prepared for extraction. The samples are heated, causing any accelerant to go from the liquid phase to the gaseous phase and then adsorbed onto a strip of activated charcoal. The charcoal will absorb any accelerant molecules that may be present. The charcoal is then used for analysis. A solvent is used to extract the accelerant from the charcoal. The resulting solution is injected into a GC/MS to identify the type of accelerant used.

When a piece of evidence is brought to the laboratory and no other units are able to analyze it, the Trace Evidence Unit will often receive that evidence and using a variety of appropriate laboratory techniques, identify or compare those items, providing valuable investigative information.