Introduction
One of the major activities by police at a crime scene is to search for items of evidence that have some probative value in the crime at hand. The crime scene, in the broad sense, includes more than the direct location where the crime occurred. It may include other physical locations where evidence has been generated. For example, a person may have been killed in one place, the body dumped in another location and the car that transported the body may have been left in a third. All of these are part of the crime scene and all of these contain evidence which has potential to help solve the crime.
Requirements for Preservation of Evidence
Every country that has a rule of law requires that evidence be preserved in the same condition as when it first appears at the crime scene, allowing for changes that take place during analysis of the evidence by the crime laboratory and normal degradation with time. There are two major reasons for this requirement. First, if the evidence is not properly preserved, it may be rendered unfit for analysis by forensic scientists and may lose whatever value it may have had in helping to solve the crime. This may also mean that there may not be enough left for analysis by the defense. This failure to preserve evidence, sometimes referred to as spoilage, may render the evidence inadmissible at trial.
The second major reason for the preservation requirement is that the court must be assured that the evidence is authentic; that is, it is the same exact piece of evidence that was seized at the crime scene. If the evidence is allowed to spoil, it may lose its identifying characteristics.
Chain of Custody
Besides preservation of evidence against spoilage, there are other means to achieve proper authentication. Chief among these is documentation of the evidence. Part of this process is included in the general procedures of recording the crime scene, but there is also the chain of custody. The chain of custody can be thought of as both a process and a document.
Chain-of-custody process
The process that is used to authenticate evidence starts with identification. When evidence is seized by a police officer, there is a requirement that the evidence (or in many cases, its container) be labeled with name or initials of the recovering officer, date and time of recovery, etc. There may also be a unique case and item identifier that is affixed to the evidence. These identifiers are also used to track the evidence throughout the police department and become a part of the record of the crime scene.
The second step in the chain of custody process is proper packaging of the evidence. How a particular piece of evidence is packaged depends upon the nature of the item and the need to preserve it. Nonetheless there is a general requirement that all evidence be packaged in tamper-evident containers. Note that this requirement does not mean tamper-proof. There is really no such thing as a tamper-proof container. Any container can be breached. The requirement is that, if a container is opened improperly or without authorization, then the tampering will be evident to people who sealed the container originally. The requirement serves to help ensure that the evidence is authentic -that it is the same evidence that was originally seized and that it is in the same condition throughout.
The next step of the chain of custody occurs when the evidence is transported to the crime laboratory. In most cases, evidence is submitted to the laboratory accompanied by a request for laboratory analysis. This form describes the evidence and the type(s) of analysis requested and contains the time, date and location from which the evidence was recovered, as well as whatever identifiers that the police have associated with the evidence. Forensic science laboratories do not, as a rule, rely upon any of these identifiers for tracking the evidence through the laboratory system because laboratory personnel have no way of knowing the source of these identifiers and to what extent they are unique. Instead, the laboratory will assign its own case and item numbers to each item of evidence. This may be done through physically writing the laboratory numbers on each piece of evidence or container, or by use of bar-code labels. This practice is increasing in popularity, at least in the United States, because it makes evidence-tracking through the laboratory system more convenient. When an analyst takes possession of the evidence, the bar-code is scanned and the data entered into a computer. The location and status of any piece of evidence can thus be instantaneously determined by querying the computer. The important thing is that each item of evidence is identified with a unique number and that all of the items in a given case are tied together by the laboratory case number system.
When an evidence container is opened and the evidence examined by a forensic scientist, that person must label each piece of evidence with the laboratory’s case number and item number and affix his or her name or initials and the date and time that the evidence was opened. When the analysis is complete, the evidence must be resealed in a tamper-evident manner. This is often accomplished by the use of evidence tape, which is specially designed to stick tightly and shred when attempts to remove it are made. When any type of tape is used, the analyst should put his or her initials and the date and time across the tape so that part of the writing is on the tape and part is on the package. This ensures that, should the tape be removed, it will be difficult to replace without leaving evidence of its removal.
This practice of police departments and crime laboratories of using two or more numbers to a piece of evidence may make the evidence look confusing with its profusion of numbers, initials, dates and times, but this is necessary to maintain the chain of custody. Each party who handles the evidence can use the appropriate markings to identify the evidence.
Chain-of-custody document
Along with the process that establishes the identify and integrity of the evidence throughout its journey from the crime scene to the courtroom, there is also a chain-of-custody document that accompanies the evidence. This is a form that describes the evidence, including the submitting agency and unique case identifiers. It also includes places where one signs and dates the document, signifying possession and control of the evidence. Every time the evidence changes hands, there is a set of signatures of the person giving up custody and the person accepting custody. With this record, any official can instantly determine who had custody of this evidence and during what period of time. The chain-of-custody document accompanies the evidence throughout its journey through the criminal justice system.
The US Constitution and other similar documents throughout the world provide that a person accused of a crime has the right to confront witnesses. Anyone who has come into possession of evidence is a witness in the broad sense and may be called to court to testify as to the fate of the evidence while in his or her possession. This means that a property clerk or intake officer in a crime laboratory who accepts evidence and then passes it on to a forensic scientist, without so much as opening up the container, may be called to court to testify. The chain-of-custody document provides the record that court officials can use to compel an appearance in court from anyone on the chain of custody.
Some police and crime laboratory systems use a separate paper for the chain of custody. This has multiple copies so that each time the evidence changes hands a copy of the chain may be given to the person who last had custody of it. An increasingly popular method of developing a chain-of-custody document is to incorporate it on to an evidence container. Many evidence bags today come with a chain-of-custody form imprinted on the front. In the case of plastic bags, the part of the bag containing the form is made of a ‘writeable’ plastic. In other cases the form may have an adhesive backing that can be used to affix the form to any container. This incorporated form has the advantage of accompanying the evidence automatically, without affixing another form to it. Xerox copies can be made of the form each time the evidence changes hands so as to provide a receipt for those who handled the evidence.
Evidence Collection
The saying ‘garbage in, garbage out’ has often been applied to computer programs, but should also be kept in mind when considering the collection of evidence from a crime scene. The crime laboratory can only analyze what it receives. Evidence that is not protected from spoilage or adulteration may not be in a condition to provide useful information. Also, the problem of insufficient material for analysis can be a major problem in crime laboratories. In many cases, evidence is pretty useless unless there is something to compare it against, so the proper collection of known standards or exemplars is very important. Each of these points is considered in more detail below.
Proper protection of evidence
The methods used to protect evidence vary with the evidence type. Obviously, evidence that is to be searched for fingerprints must not be handled with bare hands, and extreme caution must be exercised in handling it and transporting it to the laboratory. One often sees television detectives picking up a gun by shoving a pencil into the barrel and lifting it, to avoid handling it with bare hands. This practice may alter the markings on the inside of the barrel, making test-fired bullet comparisons with bullets obtained from the crime scene difficult or impossible.
Blood evidence and other human tissue must be handled with care. If fresh or wet blood or bloody garments are packaged in an airtight container, the risk of putrefaction is great and the evidence may be spoiled. In addition bloody garments must be packaged separately to avoid cross-contamination. When handling human evidence, one must also be concerned about transmission of blood-borne diseases such as hepatitis or AIDS.
The cross-contamination problem may apply to many types of evidence. Whenever possible, different items of evidence should be packaged separately to avoid the chance of cross-contamination or the exchange of evidentiary material.
Trace evidence calls for special collection methods. Because there may be very little of this evidence and it may be difficult to see, it can easily be lost. Specially equipped vacuum cleaners containing changeable filters can be used to search for trace evidence over a large area. For smaller areas, trace evidence can be collected by tape lifting. Care must be exercised with tape lifting of certain types of evidence. For example, if paint chips are affixed to tape, the muselage in the tape may adhere to the paint, thus contaminating it. Garments may be searched for trace evidence by brushing them in a clean room on to a large paper surface or into a large funnel.
Sample sufficiency
When it comes to direct, crime scene evidence, one must make do with what is at hand. More cannot be manufactured. If a scene item is limited in size, the crime laboratory will have to live with it and all crime laboratories have provisions for the analysis of sample-limited items. Often, however, insufficient samples are collected by crime scene investigators because of a lack of proper training. This training may be partially the responsibility of forensic scientists, who need to make sure that investigators are aware of the problems that arise with insufficient samples.
Known samples
The collection of exemplars presents its own set of problems. The aforementioned problem of insufficient samples should not be a consideration here. The criminal investigator should be able to collect more than enough material to satisfy the forensic examiners. This may also be a matter of training, so that the investigator is taught to know how much is enough and where to collect exemplars. For example, handwriting exemplars must be collected in certain ways, making sure that, to the extent possible, the writing implements and conditions of collection mimic those of the questioned document. Human head hair exemplars must be collected by combing, not pulling or cutting, from representative areas of the head, and as many as practical must be gathered. Test firings of weapons should be done with the same type of ammunition as the questioned bullets, etc. Each type of evidence has its own peculiar requirements for exemplar collection. Evidence collectors need to be aware of these in order to maximize the chances of successful examination of the evidence.
Negative controls
For certain types of evidence, the presence of proper negative controls is critical to successful interpretation of the evidence. For example, suppose that some suspected blood is found on a carpet near the victim of a homicide. Certainly the stained carpeting would be sent to the crime laboratory for analysis of the blood, but that is not sufficient. Some of the carpeting that has no stain on it, and which is in close proximity to the stained area, must also be collected. It is necessary for the forensic scientist to make sure that there is nothing in the carpeting that could affect the results of the analysis of the blood. The only way this can be determined is by having unstained carpet available for comparison.
