Physical and biological evidence

Evidence can be classified as Physical or Biological. Physical evidence usually comes from a non-living origin and includes finger prints, tire tracks, shoeprints and fibers. Biological evidence includes bloodstains and DNA. Locard´s exchange principle states that perpetrators always leave physical and biological evidence at the scene of the crime, and take something away with them. Such evidence is usually found in very small quantities, and is called trace evidence.

Whatever evidence is found, it is vital to establish a secure chain of custody. Each person handling the evidence from its collection to its appearance in court must sign for it. It is important that evidence is not contaminated. It must be preserved carefully, since items may deteriorate if there is a long interval before the criminal trial begins.

Some kind of physical evidence such as tire tracks and shoeprints are not enough to convict, because they do not prove the individual´s involvement. This type of evidence is known as class evidence. Class evidece is valuable if several different types of evidence can be found together. For example, finding various fibers and shoeprints is more valuable than finding onlye one fiber. Then they can be combined to identify the perpetrator.

1 DNA is a kind of (biological / physical) evidence 2 The man left (fingerprints / tire marks) when he touched the window 3 Evidence must be (contaminated / preserved) or else it will not last until the trial 4 The police need more (trace / class) evidence to convict the suspect

5 The (fibers / bloodstains) came from a wool jacket 6 Investigators wear protective clothing to avoid (preserving / contaminating) evidence

1 Did the investigators find any physical evidence? A Yes, there were shoeprints under the broken window B No, the chain of custody has not been established. 2 What trace evidence was found in the suspect´s home? A The cold weather helped preserve the crime scene B The police found mud from the crime scene in his apartment

3 Why are tire tracks so important to the case? A The paint on the car matches the one that left the accident B They can be traced back to the tires on the suspect´s vehicle 4 How do the police know the perpetrator has brown hair? A They found several hairs at the scene of the crime B The evidence shows he wears a size 11 shoe