Evidence at crime scenes may include:

1. Biological samples such as DNA from blood, semen, saliva and breath, hair, fingerprints and body part prints, urine, teeth;
2. Fibres such as pieces of material torn from clothing, or pieces of weapons broken during an attack;
3. Photographs, videos, drawings and draft of plans;
4. Documentary evidence such as receipts, travel tickets or bank statements;
5. Trace evidence such as fibers, soil, vegetation, glass fragments;
6. Digital evidence such as cell phone records, Internet logs, email messages.

Blood stains or smears are often found in crimes of violence, either on weapons or clothing, or deposited at the crime scene itself. Blood stains are biological evidence and care must be taken to avoid unnecessary exposure to a biological hazard. Proper precautions are recommended in handling and packaging any biological evidence.

Seminal stains are often found in sex offences such as rape, sodomy and incest. The presence of seminal stains confirms that sexual activity took place. However, ejaculation is not a necessary element of proof in any of these crimes. It is not unusual for ejaculation not to occur during the sexual assault. The presence or absence of seminal stains is useful in corroborating or disproving a victim's account of the crime.

Clothing, carpeting, ropes, cord, insulation and upholstery fabrics are composed of various types of fibers which can be transferred from criminal to victim or crime scene and vice versa.

In some burglary cases, they may be found caught in torn screens, broken glass, or other locations. Burglars often leave fibers from their clothing at the point of entry and pick up insulation fibers or other trace evidence from the scene of crime.

Examination of fibers can normally be conducted to determine the type or color of fiber. Such examinations may also indicate the type of garment or fabric from which they originated.

Windows, bottles and headlights are frequently broken in burglaries, robberies, assaults and hit and run cases. Occasionally, sizable pieces are carried away by the suspect that can be physically matched with one of the fragments at the scene of crime.

Paint chips and smears are frequently present on tools used to pry open doors and windows or to strike painted surfaces. Hit and run accidents involving two vehicles often result in a mutual transfer of victim and suspect paints. Particles of paint are commonly recovered from the clothing of victims of high-speed vehicle accidents.

Shoe prints or impressions are frequently present at crime scenes. They can provide a direct positive link between a suspect and the crime scene. Shoe prints may be deposited on any hard, smooth surface such as linoleum, cement, desktops, chairs, doors, or loose papers scattered over the floor.

Fingerprints are the most frequently found physical evidence at a crime scene. Present on the summits of the friction ridges on the skin are very minute sweat pores, which are constantly sweating. This sweat, or oil, grease and other substances, present on the hands or fingers, will adhere to the raised portion of these ridges. And when an object is touched, it will leave a recording of those ridges on the object.

Tool marks will most often be found in burglaries, thefts from vehicles and illegal entries. Pry bars, screwdrivers, pliers, bolt cutters, and hammers usually produce the impression-type tools marks. Bolt cutters, tin snips and pliers frequently leave tool marks on cut locks, windows and doors, etc. Knives used to cut wires or rubber pipes leave identifiable tool marks over the wire or rubber.

The use or possession of firearms is found in a significant number of crimes. Firearms, as well as discharged or intact ammunition, are often important evidence in any firing investigation. In fact, it is almost impossible to get a conviction in shooting cases without the discovery of such evidences. A forensic ballistics expert matches bullets, fragments, and other evidence with the weapons of alleged suspects or others involved in a case.

Any item (clothes or footwear) containing soil, wood, natural resources like minerals or other vegetative matter may help in linking a person or object to a particular location; for example, soil imbedded in shoes and found on garments. Areas of exposed soil in or around a crime scene raise the possibility of soil being transferred to the suspect's shoes or tyres upon entry or departure. Impacts in hit and run accidents frequently result in the deposition of dirt or mud on the street.

Locard’s Exchange Principle

The key principle underlying crime scene investigation is a concept that has become known as Locard’s Exchange Principle. It states, “Whenever someone enters or exits an environment, something physical is added to and removed from the scene.”

“It is impossible for a criminal to act, especially considering the intensity of a crime, without leaving traces of this presence.” Edmond Locard

Locard believed that no matter where a criminal goes or what a criminal does, he will leave something at the scene of the crime. At the same time, he will also take something back with him. A criminal can leave all sorts of evidence, including fingerprints, footprints, hair, skin, blood, bodily fluids, pieces of clothing and many more. By coming into contact with things at a crime scene, a criminal also takes part of that scene with him, whether it is dirt, hair or any other type of trace evidence.

Locard advocated the application of scientific methods and logic to criminal investigation and identification. His work formed the basis for what is widely regarded as a cornerstone of the forensic sciences. By recognizing, documenting, and examining the nature and extent of this evidentiary exchange, Locard observed that criminals could be associated with particular locations, items of evidence, and victims of crime.

Paul L. Kirk expressed the principle as follows: “Wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as a silent witness against him. Not only his fingerprints or his footprints, but his hair, the fibres from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects. All of these and more bear mute witness against him. This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong, it cannot perjure itself, and it cannot be wholly absent. Only human failure to find it, study and understand it can diminish its value.”

Trace evidence is based on Locard’s Exchange Principle, which contends that every contact, no matter how slight, will leave a trace. Objects or substances contacting one another, and leaving a minute sample on the contact surfaces normally cause the trace.

1. There are many ways that fibres can be transferred in the commission of a crime. They can transfer from a fabric source like a carpet, bed, or furniture at a crime scene. This is called direct or primary transfer of forensic fibres.
2. A secondary transfer occurs when already transferred fibres on the clothing of a suspect transfer to the clothing of a victim. For instance, if fibres from a carpet in the suspect’s bedroom are transferred to the victim, without the victim ever having been in that bedroom.

Understanding the mechanics of primary and secondary transfer is important when reconstructing the events of a crime. The main point is that some apparently foreign object or piece of material has been brought to a crime scene and tracing its origin can assist in an arrest and conviction. Similarly, finding some trace from the victim or crime scene on a suspect can also have a strong impact on a case.