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Law enforcement officers and detectives use everything they have at their disposal to solve crimes and assure a conviction of the perpetrator. One type of evidence often collected at a crime scene is impression evidence. Although the most common examples are footprints or tire marks, impression evidence includes any object or material that takes on the form of another object as a result of contact between the two.
Just as criminals often leave fingerprints at the scene of a crime, they are even more likely to leave footprints. While a person can wear gloves to avoid leaving fingerprints, it is almost impossible to get to the crime scene without walking around it at some point. Although fingerprint evidence is more helpful in the sense that it can provide an exact match due to the fact that no two fingerprints are the same, footprint impressions can tell investigators things such as what type of shoe the suspect was wearing, what size shoe he or she wears, and how much he or she weighs. They can also often indicate whether the person who made them limps or was carrying anything at the time.
Tire impressions are another type of impression evidence that is often used in criminal investigations. A tire impression can tell investigators what type of tire it came from, who manufactured the tire, and what type of car typically uses that type of tire. Tool marks are sometimes excellent clues when investigators are trying to solve a crime. A tool may have been used to break into a building, or even as a weapon. By lifting an impression, detectives may get a useful lead.
In order to use the clues left by impression evidence, a latent impression or a cast of the impression is usually taken at the crime scene. When the impression is a two-dimensional impression, such as muddy boots that walked across a tile floor, powder is applied and a latent print is lifted in much the same way that fingerprints are gathered. When the impression left is three-dimensional, such as tire marks in mud or footprints in the snow, then a cast is made of the impression and allowed to dry before lifting it out to study. Tools often leave an impression behind when used as well that can be lifted by either a latent or cast method.
Once the impression has been preserved, it can be studied by forensic scientists. In some cases, the impression gives investigators a vital clue in the apprehension of a suspect. In addition to leading to the arrest of a suspect, an impression may also be used at trial to convict a defendant by comparing the impression to actual evidence found in the suspect's possession, such as the boot or tire that made the impression.