How to Process a Crime Scene: Documentation and Evidence Collection

Processing a crime scene step by step means moving through a fixed sequence: establish legal authority, protect yourself, secure the perimeter, walk through, document, search, collect and package, maintain chain of custody, and conduct a final survey before release. Each step protects the one after it. Skip or rush any of them and evidence that would have convicted someone can end up suppressed, contaminated, or thrown out.

Step 1: Establish Legal Authority to Enter

Nothing else matters if investigators cannot lawfully be on the scene. The Fourth Amendment protects people from unreasonable searches and requires warrants that specifically describe the place to be searched and the items to be seized, backed by probable cause.¹ Under the Federal Rules of Criminal Procedure, a magistrate judge issues a search warrant after reviewing an affidavit or sworn testimony showing probable cause that evidence of a crime will be found at the location.²

A few narrow exceptions let officers act without a warrant. If they reasonably believe someone inside needs emergency help, they can enter and search for victims. They can also sweep the premises to check whether a suspect is still present or additional victims are inside. Anything spotted in plain view during those lawful emergency actions can be seized without a separate warrant.³

Once the emergency ends, so does the justification for being there without a warrant. Officers can secure the premises for a reasonable time while they obtain one, but they cannot keep searching. The Supreme Court explicitly rejected a general “murder scene exception” in Mincey v. Arizona, holding that the seriousness of a crime does not by itself create the emergency needed to justify a warrantless search.⁴ Skip the warrant without a recognized exception and every piece of evidence is at risk of suppression.

Step 2: Put On Protective Equipment

Crime scenes expose investigators to bloodborne pathogens, chemical hazards, sharp objects, and sometimes airborne contaminants like fentanyl powder. Nobody processes anything until the scene is confirmed safe and everyone is properly equipped.

OSHA’s Bloodborne Pathogens standard requires employers to provide personal protective equipment at no cost whenever occupational exposure to blood or other infectious materials is reasonably anticipated, and it must prevent blood or infectious material from reaching skin, eyes, mouth, or clothing under normal working conditions.⁵ At minimum that means:

Nitrile gloves whenever hand contact with blood or contaminated surfaces is possible. Single-use gloves cannot be washed or reused, and investigators should change them between items to avoid cross-contamination.
Goggles, glasses with side shields, or chin-length face shields when splashes or droplets of blood are a risk.
Gowns, aprons, or coveralls when body exposure is likely, with shoe covers or boots added for heavy contamination.
Masks or respirators when airborne hazards are present, including scenes where powdered narcotics may have been released.

All protective equipment must be removed before leaving the work area, and any garment penetrated by blood must come off immediately.⁵ At scenes involving suspected synthetic opioids, the most dangerous exposure route is inhaling airborne powder, not skin contact. Alcohol-based hand sanitizers should not be used for decontamination because they can actually increase skin absorption.

Step 3: Secure the Perimeter and Control Access

First responders establish a perimeter using barrier tape, vehicles, or natural boundaries. Set it wider than what looks necessary. Shrinking a perimeter later is easy. Discovering you trampled evidence outside the original boundary is not.

Access control is the single most important function once the perimeter is up. Every person who enters a crime scene can alter it, whether by tracking in soil, shifting objects, or shedding hair and fibers. A designated officer keeps a crime scene entry log recording the name, agency, reason for entry, and arrival and departure times of everyone who crosses the perimeter.⁶ Only personnel with a documented purpose and the right protective equipment go in. That log becomes part of the case file and may be scrutinized at trial to show who had access to the evidence.

Step 4: Conduct a Preliminary Walk-Through

Before detailed documentation or evidence collection starts, the lead investigator walks the entire scene. This initial survey provides an overview, identifies threats to scene integrity, and helps determine what resources and personnel are needed.⁶

The investigator uses the established entry path to avoid contaminating undisturbed areas. The goal is to identify fragile or perishable evidence that needs immediate attention: items exposed to rain, footprints in snow, biological material in direct sunlight. Anything at risk of being lost gets documented, photographed, and collected right away rather than waiting for the full processing sequence.⁶ The walk-through also helps investigators form an early theory of what happened, which guides search patterns and collection priorities.

Step 5: Document the Scene

Documentation creates a permanent record of the scene in its original condition. Nothing gets moved or collected until this step is complete, because once evidence is disturbed the scene can never be restored. Photography, sketching, and written notes work together, since each method captures information the others miss.⁷

Photographs and Video

Crime scene photos follow a deliberate progression. Overall shots establish the scene’s location and layout. Mid-range shots show evidence in relation to surrounding objects. Close-ups capture detail on individual items, often with a measurement scale in the frame.⁷ Video may supplement stills at complex scenes where movement through the space helps convey spatial relationships.

Sketches and Notes

Photographs can distort spatial relationships and distances, so sketches fill that gap. A rough sketch drawn at the scene captures measurements between objects, walls, and reference points, which become the basis for formal, scaled diagrams later.⁷ Written notes run continuously throughout the investigation, recording environmental conditions like temperature and lighting, the time of each action, and observations a camera cannot capture, such as odors or sounds. Notes should begin the moment responding officers arrive and continue until the scene is released.⁸

Step 6: Search Systematically

With the scene documented, investigators begin a systematic search. The word “systematic” matters here. Unstructured searching is how evidence gets overlooked. Choose a pattern based on the scene’s size, layout, and expected evidence.

Grid: two searchers move in perpendicular parallel lines across the scene, covering each area twice. Best for outdoor scenes where thorough coverage matters most.
Strip or line: multiple searchers walk shoulder to shoulder in straight parallel lines across large open areas like fields or parking lots.
Spiral: a single searcher moves outward from a central point in concentric circles. Works well in open areas without clear boundaries.
Zone or quadrant: the scene is divided into distinct sections, each assigned to a different searcher. Useful for complex indoor scenes with multiple rooms or floors.

Beyond visual inspection, investigators use detection tools that reveal evidence invisible to the naked eye. Alternate light sources emit specific wavelengths that cause biological fluids like semen, saliva, and urine to fluoresce. Luminol, sprayed on surfaces in a darkened room, reacts with the iron in blood to produce a faint blue glow, revealing traces even after someone tried to clean up. For fingerprints, investigators dust smooth surfaces with fine powder, use cyanoacrylate (superglue) fuming on nonporous items, and apply chemical developers like ninhydrin to porous surfaces such as paper.

Step 7: Collect and Package Evidence

Once evidence is found and documented in place, collection and packaging determine whether it arrives at the lab in usable condition. The overriding principle is to prevent contamination, degradation, and cross-contamination. Every piece of evidence gets its own container, its own label, and its own seal.

Biological Evidence

Blood, saliva, and other biological material must be air-dried before packaging. Sealing wet biological evidence in an airtight container promotes mold and bacterial growth that can destroy DNA. After drying, biological items go into breathable containers like paper bags or cardboard boxes rather than plastic. If evidence cannot be fully dried at the scene, transport it in a leak-proof container to a drying facility, then repackage it in paper and store as cold as possible.⁹

Sharp Objects and Firearms

Knives, broken glass, needles, and similar items go into rigid, puncture-resistant containers for handler safety. Firearms should be unloaded before packaging whenever it can be done safely, with the weapon’s condition and any ammunition documented separately. Avoid handling items by surfaces likely to carry fingerprints or DNA.

Digital Evidence

Almost every crime scene now involves electronic devices, and they require a different handling approach. The core principle is that nothing done during collection should change the data on the device.¹⁰

Computers found powered off stay off. Turning one on can alter timestamps, overwrite temporary files, and trigger startup processes that destroy volatile data. Document all connected cables and devices, label each connection point, disconnect power from the back of the machine, and secure all cables. Tape over drive slots and the power switch to prevent accidental activation during transport.¹⁰

A running computer is harder. If the screen shows data being deleted or overwritten, pulling power immediately is recommended to preserve what remains. In a typical desktop environment, disconnecting power from the back of the machine preserves information about the last user login and session time.¹⁰ Investigators without specific digital forensics training should not attempt to explore a computer’s contents beyond recording what is visible on the screen.

Mobile phones and smartphones stay in whatever power state they are found in and get wrapped in radio-frequency shielding material like a Faraday bag or aluminum foil to block incoming signals. Without shielding, a remote wipe command or incoming messages could alter or destroy evidence.¹⁰

Sealing and Labeling

Every evidence container is sealed with tamper-evident tape. The person sealing it marks the tape with their initials and the date, with markings crossing from the tape onto the packaging so any attempt to open and reseal the container is visible. Labels include at minimum the case number, item description, collector’s name, and date and time of collection.

Step 8: Maintain the Chain of Custody

The chain of custody is the documented trail showing who had possession of each piece of evidence, when they received it, and what they did with it from collection through laboratory analysis to courtroom presentation. Every person who handles an item signs a log entry recording the transfer.¹¹

The purpose is legal: proving the evidence presented in court is the same evidence collected at the scene and has not been altered, contaminated, or swapped. A proper log includes the geographic location where the item was found, a photograph of that location, and a signed entry from each person who handled it after.¹¹

A gap in this record is where cases fall apart. If a defense attorney identifies a period when nobody can account for where the evidence was or who had access to it, the evidence becomes vulnerable to a suppression motion. Even well-collected evidence with strong forensic results can lose its legal weight because of sloppy paperwork.¹¹

Step 9: Final Walk-Through and Scene Release

Before releasing the scene, the lead investigator conducts a final survey. It mirrors the preliminary walk-through: a deliberate pass through the entire area to confirm all evidence has been collected, all documentation is complete, and no equipment has been left behind.⁶ Address any gaps before release, because once the scene is turned back over, re-entry will likely require a new warrant.

After release, collected evidence is transferred to a forensic laboratory for analysis. Include a narrative or a copy of the police report with the submission so lab staff understand the context and can properly prioritize items.¹² Marking each item with a priority notation helps the lab decide which exhibits to process first, which matters when suspects are in custody and charging deadlines are approaching.

1
Congress.gov. Fourth Amendment – Overview of Warrant Requirement
2
United States Courts. Federal Rules of Criminal Procedure – Rule 41
3
Federal Law Enforcement Training Centers. A Murder Scene Exception to the Fourth Amendment Warrant Requirement
4
Library of Congress. Mincey v Arizona, 437 US 385
5
Occupational Safety and Health Administration. 29 CFR 1910.1030 – Bloodborne Pathogens
6
Office of Justice Programs. Crime Scene Investigation – A Guide for Law Enforcement
7
National Institute of Justice. Crime Scene and DNA Basics for Forensic Analysts – Section: Methods of Crime Scene Documentation
8
National Institute of Standards and Technology. OSAC 2023-N-0002 Standard for Scene Documentation Procedures
9
National Institute of Justice. What Every Investigator and Evidence Technician Should Know About DNA Evidence – Section: Air-Dry Evidence
10
Office of Justice Programs. Electronic Crime Scene Investigation – A Guide for First Responders
11
National Institute of Justice. A Chain of Custody – The Typical Checklist
12
National Institute of Justice. Evidence Prioritization and Submission to the Laboratory