The key to any successful criminal investigation and prosecution is the quality of evidence obtained at the crime scene. The more evidence collected, the greater the likelihood of a conviction. Crime scene investigators are highly skilled in the investigation and collection of evidence, and they often have to be on the lookout for numerous types of evidence. Here are some of the basic evidence types found at a crime scene.
Physical evidence is one of the most common types of evidence found at a crime scene. Physical evidence consists of the actual physical objects found at the scene. This can mean large items such as damaged cars, broken glass or smashed doors. It also includes items that are minuscule in size, such as hair or clothing fibres. An investigator may also collect weapons such as knives or guns, or fired bullets and spent casings. Depending on the scene, physical impressions may also be found, including tire tracks or footprints. A suspected burglary may lead the investigator to look for tool marks on the doors or windows. Finally, physical evidence also includes fingerprints and lipstick impressions left on glasses or cigarettes. If it can be touched, picked up or moved it constitutes physical evidence.
Trace evidence is a subset of physical evidence consisting of evidence so small it may not be readily apparent but is still found in a sufficient quantity to be measured. Often a microscope or ultraviolet light may be needed to see the trace evidence. Trace evidence can consist of trace amounts of blood found on a wall or on a knife blade. It may also include wood splinters from a bullet hole, or minute amounts of dirt on a pair of shoes.
Biological evidence is any bodily fluid or other bodily tissue. Technological advances in the last 30 years have placed an increased emphasis on the collection of biological evidence at crime scenes. Through DNA testing, biological evidence can provide for a near-positive identification of the donor. Biological evidence may consist of blood, semen, bone fragments, skin, saliva or hair.
While drug evidence is technically physical evidence, it is usually classified by itself. Drug evidence consists of any legal or illegal substance controlled by law. The evidence may consist of large quantities of the drug, or trace amounts such as powder on a spoon or resin in a pipe. It also includes evidence of the use, ingestion, manufacture or distribution of controlled substances. Consequently, syringes, scales, pipes and plastic bags are considered drug evidence if they appear to have been used to ingest, manufacture or distribute drugs.
There are a variety of other types of evidence found or collected at a crime scene that may not fit into the other categories. Toxicology reports are used to determine what substances were in a person at the time of the crime. Crime scene reconstructions are performed at the scene to try and determine bullet trajectories or blood spatter. Interviews with witnesses, suspects and victims also constitute evidence, and most initial interviews occur at the crime scene.