Preserving the Chain of Custody
This is the most important part of processing a crime scene. When you collect a piece of evidence, you must indicate on the package the following:
The case number
An identification number
A brief description of the item
The time, day, date, and place of collection
The name of the person who collected it
When you collect evidence it's considered to be in your custody. When you deliver it to the crime lab, the person receiving it must indicate the same — case number, time, day, date, and place of collection — and sign it. She now has custody. This must be repeated when the sample is examined, and any time it's transferred from one person to another. This process is called the chain of custody, and is the documented history of this piece of evidence. Most importantly, evidence must reach court with the chain unbroken. If a question is raised concerning custody, suspicions will also be raised as to tampering, or accidental alteration. Either way, it throws suspicion on your evidence — and your testimony concerning it.
You'll also find yourself in possession of evidence apart from crime scenes — court records, video of subjects and the like. You must be able to account for each person who touched that evidence and where it's been stored, in order to protect the chain of custody. Breaking the chain is cause for inadmissibility of evidence.
Let's say that someone who works for you records video of a workers compensation subject, and brings it to the office. A technician downloads the recording onto a DVD and stores it in a locked file cabinet. Each one of these people must sign and date that they've had this evidence in hand. If there's ever a time when evidence can't be accounted for, the chain is broken.
Using The Scientific Method
Investigators use different methods for putting evidence together to prove facts. The scientific method is used most often.
State the problem — state crimes committed and in what jurisdiction — if working for attorney or with police, match state and local law to the crimes
Form the hypothesis — determine possible suspects based on information gathered from victim, witnesses, physical evidence, and possible motive
Collect data — search records, locate additional witnesses — obtain physical evidence from the scene and compare it to the suspects' physical evidence if possible — interview the witnesses again based on data you've discovered
Interpret data to test hypothesis — evaluate evidence and focus on the most likely suspect (police provide Miranda at this point), interrogate suspect
If data supports hypothesis, collect additional data — review evidence again — seek new evidence — review whether this data supports or disproves your original hypothesis — if not alter your hypothesis (many falter at this point — they don't want to begin again)
With new hypothesis, go through steps again
Draw conclusions — when your data and hypothesis match as much as possible, draw a conclusion as to what, where, how, and why the crime happened
Develop a theory — develop your theory as to who committed the crime from these conclusions
This is a simple example, but it provides an overview of one means of getting to conclusions and theories that can be presented in court. Investigation is an unusual profession. It exists along a continuum of art and science, and although it's still more of an art, both disciplines play a large part. Some experts insist that, with recent advancements in technology, investigation is moving farther along the continuum toward science, and will continue to do so. In this case, investigators, including PIs, will need to become much more acquainted with the scientific method.
