A crime scene investigator collects, documents, and preserves the physical evidence found at a crime scene so forensic specialists can analyze it and prosecutors can use it in court. The work is hands-on and scientific: photographing blood spatter, lifting fingerprints, bagging shell casings, swabbing for DNA. Crime scene investigators do not interrogate suspects, chase leads, or make arrests. Their job starts when they arrive at the scene and ends when the evidence reaches the lab with its paperwork intact.
What a CSI Does, and What a Detective Does
Television tends to merge the two jobs. In most departments they are separate. Detectives handle the investigative side: interviewing witnesses, questioning suspects, building the case narrative, and making arrests. Crime scene investigators handle the physical side: identifying what matters at the scene, collecting it correctly, and getting it to the lab without contamination.
Whether the CSI is a sworn officer or a civilian employee depends on the agency. Large departments often hire civilians with scientific training and no arrest authority. Smaller agencies tend to use sworn officers who have completed specialized forensic training on top of their standard police work. At the scene, the function is the same.
Securing and Documenting the Scene
The first task at any scene is locking it down. Every person who walks through risks destroying something — a shoe smears a blood trail, a hand grabs a door handle, a foot kicks a casing under a table. CSIs or the first responding officers set a physical perimeter with tape, cones, or existing walls and fences, then control who enters and exits.1FBI. Crime Scene Investigation: A Guide for Law Enforcement Everyone who crosses that line gets logged. The log is what later blocks a defense attorney from arguing that an unknown person contaminated the evidence.
Documentation begins before anything is touched. It runs on three overlapping tracks: photography, sketching, and written notes.1FBI. Crime Scene Investigation: A Guide for Law Enforcement Photographs move from wide shots of the whole area down to close-ups of individual items, usually with a measurement scale beside the evidence. Sketches map spatial relationships: where the body lay relative to the door, how far a casing landed from the window, the distance between bloodstains. Written notes capture what cameras miss — temperature, smells, sounds, time of arrival, and any reason the investigator deviated from standard procedure.
Scenes get cleaned up or released. The photographs, sketches, and notes become the permanent record of a place that no longer exists in its evidentiary state. Years later at trial, this is what the jury sees.
Collecting and Packaging Evidence
Once documentation is complete, collection begins. Evidence varies by crime, but most of it falls into a few categories:
- Biological evidence — blood, saliva, hair, skin cells, sexual assault evidence. These are the raw material for DNA analysis and often the strongest link between a suspect and a scene.
- Latent prints — fingerprints and palm prints invisible to the eye, revealed with dusting powder, chemical fuming, or alternate light sources. Footwear and tire impressions belong here too.
- Trace evidence — fibers, soil, paint chips, glass fragments. Small materials that transfer between people, objects, and environments.
- Ballistics — firearms, spent cartridge casings, bullets, and shot patterns, used to determine the weapon, the shooter’s position, and the number of shots fired.
How evidence is packaged matters as much as finding it. Biological material goes into paper bags or cardboard boxes after being air-dried, because sealing wet biological material in plastic promotes bacterial growth and degrades DNA.2National Institute of Justice. Proper Evidence Collection and Packaging Plastic is used only for short transport when excessive body fluids create a contamination risk, and the items get repackaged in paper as soon as possible.3National Institute of Justice. Collecting DNA Evidence at Property Crime Scenes – Packaging for Transportation Firearms are rendered safe and packaged separately from ammunition. Each item gets its own container to prevent cross-contamination.
Investigators sometimes run presumptive tests at the scene — a quick chemical test to indicate whether a stain could be blood, for example. Those are screening tools, not confirmations. The actual forensic analysis happens later at the crime lab.
Handling Phones and Other Digital Devices
Modern scenes routinely include smartphones, laptops, tablets, security cameras, and smart home devices. A phone is not a shell casing. If it is still connected to a network when collected, it can be wiped remotely while sitting in an evidence bag. Isolation comes first.
The standard practice is placing the device in a Faraday bag, a container lined with material that blocks radio signals. This stops incoming commands from reaching the device while preserving its contents for forensic examination.4Scientific Working Group on Digital Evidence. Best Practices for Mobile Devices Evidence Collection and Preservation Handling and Acquisition Airplane mode was once enough, but newer operating systems sometimes leave Bluetooth or Wi-Fi active even in airplane mode, so physical signal blocking is preferred. One wrinkle: Faraday bags drain batteries faster because the phone keeps searching for a signal it cannot find, and investigators have to keep the device powered without running a charging cable through the shielding.
Chain of Custody and Court Testimony
Every piece of evidence a CSI collects carries a paper trail documenting who handled it, when, and why — from the moment of collection until the moment it is presented in court. This is the chain of custody, and it exists to prove the evidence has not been tampered with or swapped. If the prosecutor cannot show an unbroken chain, the judge can exclude the evidence or instruct the jury to give it less weight.5National Institute of Justice. Law 101: Legal Guide for the Forensic Expert – Chain of Custody A single gap, one missing signature or one undocumented transfer, can unravel months of work.
CSIs regularly testify at trial. The testimony is methodical, not dramatic: how you arrived at the scene, what you observed, which protocols you followed for each item, and how you maintained the chain of custody. Defense attorneys press hard on procedure. Deviations get found. Incomplete notes get exploited. The quality of documentation at the scene often decides how well the evidence holds up years later on the stand.
Investigators also carry a legal duty to disclose evidence that might help the defense. Under Supreme Court precedent, law enforcement must turn over exculpatory material that could reasonably affect the outcome of a trial or the credibility of a government witness. A CSI who finds evidence pointing away from the suspect has the same duty to preserve and report it as evidence pointing toward them. Selective collection can result in case dismissal and personal liability.
Intentionally altering, destroying, or concealing evidence is a serious crime. At the federal level, tampering with evidence related to a federal investigation carries up to 20 years in prison.6GovInfo. 18 USC 1519 – Destruction, Alteration, or Falsification of Records in Federal Investigations State penalties vary but routinely include felony charges with multi-year prison terms.
Working Conditions and Hazards
The job puts you in contact with hazardous materials as a matter of routine. Bloodborne pathogens are the most common concern — HIV, hepatitis B, hepatitis C. Federal workplace safety standards require protective equipment whenever contact with blood or other infectious materials is reasonably anticipated: gloves at a minimum, plus masks, eye protection, and protective clothing when splashing or spraying is possible.7CDC. 29 CFR 1910.1030 – Bloodborne Pathogens Decomposing remains, chemical residues from clandestine drug labs, and biohazard scenes add further risk.
Conditions are demanding. Nearly half of forensic science technicians report regular outdoor exposure in all weather, and most work more than 40 hours per week. Scenes do not wait for convenient timing. Call-outs come at 3 a.m., on holidays, in rain, in abandoned buildings with no power.
The psychological toll is less visible but just as real. Repeated exposure to homicides, assaults on children, and fatal accidents produces what researchers call secondary traumatic stress — hypervigilance, sleep disruption, irritability, difficulty with routine tasks. Over time it can develop into compassion fatigue or burnout. Most agencies now offer peer support, access to counseling, and structured debriefing after particularly difficult scenes.
How to Become a Crime Scene Investigator
Most positions require at least a bachelor’s degree in forensic science, biology, chemistry, or criminal justice.8Bureau of Labor Statistics. Forensic Science Technicians – Occupational Outlook Handbook Some agencies accept an associate degree paired with relevant work experience. Coursework in evidence handling, crime scene processing, and laboratory techniques gives applicants an edge. A master’s degree is rarely required for entry-level work.
On-the-job training varies. Civilian CSIs in large departments typically receive specialized instruction in forensic photography, latent print processing, and evidence collection before working scenes on their own. Sworn officers moving into crime scene work attend dedicated forensic training programs on top of the standard police academy.
The International Association for Identification offers a Crime Scene Certification, the closest the field has to a nationally recognized professional credential.9International Association for Identification. Crime Scene Certification Levels include Crime Scene Investigator, Crime Scene Analyst, and Senior Crime Scene Analyst, earned through a combination of experience, education, and an examination. It is not mandatory at most agencies, but it signals competence in a field where mistakes sink prosecutions.
The Bureau of Labor Statistics groups crime scene investigators under forensic science technicians. Median annual pay is roughly $64,000, and employment is projected to grow 13 percent between 2024 and 2034, categorized as much faster than average.8Bureau of Labor Statistics. Forensic Science Technicians – Occupational Outlook Handbook Advances in DNA analysis and digital forensics continue to expand what evidence can be recovered, which drives demand for people trained to collect it properly.