Trace evidence like hair, fibers , paint, and blood , is by its very nature readily transferred from item to another. This raises the problem of cross contamination, where the source of trace evidence found on a significant item is uncertain. The trace evidence may have attached itself to a relevant item during the crime itself, in which case it becomes significant evidence. However, it is also possible that the evidence was transferred to the item via a third party during the investigation. This would be cross contamination, and such evidence is detrimental to an investigation. When a case comes to court, expert witnesses will always be on the lookout for the possibility of cross contamination, especially when a serious crime like murder or rape is involved. The only way of avoiding cross contamination is to follow strictly controlled forensic procedures during an investigation. This means making certain that the only way trace evidence could have arrived on an item is during the crime.
For instance, supposing fibers or hair from a victim are found on the clothing of a suspect. Such trace evidence could be highly incriminating. But what if an officer comes from the scene of the crime to the suspect's home and packs up his clothes for forensic examination? It is possible that the officer picked up trace evidence from the scene of the crime and acted as an intermediary, transferring it to the clothing of the suspect. Ideally, investigators would not attend more than one scene linked to the crime—in this case, the scene itself and the suspect's home—to avoid cross-contamination of this kind. Given there is a finite number of personnel available for each crime investigation, sometimes the same officer will be involved at more than one scene. In such cases, he or she must follow a strict decontamination process between attending different crime scenes. Careful records must also be kept of the movements of investigators between different scenes.
Cross contamination could also occur if packaging and re-packaging of items is not done correctly. It is essential to package each piece of evidence separately in an unused container. Obvious sources of cross contamination should always be kept well separated. For instance, the clothing of the suspect should never be packaged or handled in the same room as that of the victim and the investigators should be able to prove this procedure was followed. Only by following such procedures to prevent cross contamination can the true value of trace evidence be revealed in court.
see also Evidence, chain of custody.