Crime scene investigation merges science with the law. A crime scene investigator needs to have a thorough knowledge of scientific observation and methods, as well as knowledge of state and federal laws regarding evidence.

A crime scene investigator will be called upon to evaluate the evidence at the scene of homicides, sexual crimes, robberies and burglaries, and home invasions, among other crimes. A CSI can be called upon on any day and at any hour; because immediacy is vital to the preservation of a crime scene, they must be capable of working with minute details at all times.

Investigators may or may not be forensic scientists, as well. If qualified in forensic science, the CSI may perform some of the follow-up laboratory evaluation away from the crime scene - assessment of hair, fibers, drugs, body fluids, firearms, and fingerprints. Whether a CSI is a forensic scientist or not, he still needs an in-depth knowledge of how forensic science works in order to properly preserve laboratory evidence as it is collected at the crime scene.

Crime scene investigators are responsible for systematically processing a crime scene by:

• Securing the scene
• Reconstructing events
• Preserving, collecting, and packaging evidence
• Photographing and/or creating video of the scene
• Drawing sketches and diagrams
• Maintaining evidence logs, photography logs, and other tracking data
• Writing detailed reports about evidentiary findings
• Attending and photographing autopsies
• Using an array of scientific equipment to collect and analyze evidence
• Testifying in court with regard to evidence collected

A crime scene investigator does not offer opinions, only scientific data. He utilizes a four-step process to accomplish this:

1. Evaluation

During the evaluation stage, the CSI assesses the overall scene during a preliminary walk-through to determine the best plan for processing it without destroying evidence. Because each crime scene is different, the investigator needs to be able to quickly organize observations and develop a plan.

2. Documentation

At the documentation stage, the CSI takes extensive photographs: wide-view pictures to depict the entire scene, mid-view pictures to show evidence in context, and close-up pictures to show details of evidence. Videos may be taken; sketches and diagrams are drawn.

3. Collection

In the collection stage, laboratory evidence is carefully handled, secured in proper storage containers, and documented in logs to preserve the chain of evidence for trial. Everything from simple tools, such as tweezers, to sophisticated scientific methods, such as luminol, may be used.

4. Analysis

At the analysis stage, the collected evidence is thoroughly evaluated in the lab. If the CSI is also a forensic scientist, he participates in this process.

Crime scene investigators work toward several goals:

• Identification of the perpetrator of the crime
• Identification of the victim of the crime (if unknown)
• Reconstruction of the crime
• Conviction of the perpetrator

A crime scene investigator does not interview witnesses, pursue the suspect, or conduct interrogations. They do accurately document every significant detail of the crime to assist the district attorney in developing a court case that will stand up to attack by the defense attorney. They work in conjunction with the detectives, providing data that assists the detectives in identifying the perpetrator. They also testify during the trial about the evidence and its collection.

The job of a crime scene investigator is highly-specialized and requires up-to-date knowledge of science and the law. It is an often gruesome, but extremely important job. Without it, many criminals might never be identified.