Evidence is collected through a systematic process of identifying, documenting, recovering, and preserving items or data so they remain reliable for analysis and decision-making. The exact techniques depend on the context, whether it’s a crime scene, a medical lab, a digital device, or an environmental site, but the core principle is always the same: capture information without altering or contaminating it. Here’s how that works across the most common settings.
Crime Scene Evidence: The Seven-Stage Process
Crime scene investigation follows a consistent sequence regardless of the type of crime. It starts with identifying the scene’s dimensions. Investigators locate the focal point and establish a perimeter large enough to contain all relevant evidence. The general rule is to make the boundary larger than you think you’ll need, since you can always shrink it later but expanding it risks losing material that’s already been disturbed.
Once the perimeter is taped off and secured, the team creates a plan. They determine what type of crime likely occurred, identify threats to the evidence (weather is a major one for outdoor scenes), and decide whether additional resources are needed. From there, investigators conduct a primary survey: walking through the scene, photographing everything, identifying potential evidence, and taking detailed notes before anyone touches anything.
The actual collection happens during the processing phase. Every item is recovered using techniques matched to its type, logged in an inventory, and packaged to prevent degradation. After processing, a secondary survey sweeps the scene again to confirm nothing was missed. Every piece of evidence gets a thorough written description that corresponds to the photographs taken at the scene.
Trace Evidence: Fibers, Glass, and Residue
Microscopic evidence requires specialized recovery techniques. According to guidelines from the National Institute of Standards and Technology, the goal is always to use the most direct and least intrusive method practical. Investigators detect trace evidence through visual searches, sometimes aided by oblique lighting, ultraviolet light, lasers, or magnification.
The main collection methods include:
- Picking: Using clean forceps to separate visible trace material from a surface, then immediately packaging it to prevent loss.
- Lifting: Pressing adhesive tape repeatedly over an item so loosely attached fibers, hair, or particles stick to it. The tape is placed on a transparent backing like a glass slide or clear plastic so the material can be viewed without removal.
- Scraping: Using a clean spatula to dislodge material onto collection paper. This is typically done in a lab to reduce contamination risk.
- Vacuum sweeping: Running a vacuum with a filter trap over an area. The filter and trap must be rigorously cleaned between each use. This method is usually saved for last because it’s indiscriminate and picks up large amounts of irrelevant material.
- Combing: Running a clean comb through a person’s hair to recover foreign material. The comb and debris are packaged together.
- Clipping: Cutting fingernails or scraping underneath them. Nails from the right and left hands are packaged separately, and the scraping tool goes into the package with the debris.
Biological and DNA Evidence
DNA samples are typically collected by swabbing a surface with a cotton-tipped applicator moistened with saline solution. The swab is then dried at room temperature (usually for about two hours if it’s wet) and sealed in a protective bag until it can be sent to a lab for extraction. Drying is critical because moisture promotes bacterial growth that degrades DNA.
Contamination prevention is especially strict with biological evidence. Collectors wear a full set of protective clothing: disposable coat, hair net, face mask, double gloves, and footwear covers. Gloves are changed between handling different items. Evidence should never be rinsed, washed, or wiped, since that destroys biological material. Even trace amounts of someone else’s DNA transferred from an ungloved hand can compromise a sample.
Digital Evidence: Copying Without Changing
Collecting evidence from computers, phones, and storage drives presents a unique challenge. Simply turning on a device or opening a file can alter timestamps and metadata, potentially invalidating the evidence. To prevent this, forensic examiners use hardware write-blockers, physical devices that sit between the storage drive and the examiner’s computer and allow data to be read but block any changes from being written back.
NIST runs a dedicated testing program that certifies these write-block devices to ensure they actually prevent data alteration. Once a write-blocker is in place, the examiner creates a bit-for-bit copy (called a forensic image) of the entire drive. All analysis is performed on the copy, leaving the original untouched. The forensic image is verified using a digital fingerprint, a unique value calculated from the data that would change if even a single bit were altered.
Medical and Laboratory Specimens
Blood, urine, and tissue samples follow strict protocols to ensure test results are accurate. For blood draws, the collection tube matters: manufacturers mark each tube with the correct fill volume, and the internal vacuum pulls in the right amount when it’s allowed to exhaust naturally. For patients with fragile veins, smaller tubes (1 to 5 milliliters) with low vacuum pressure and finer needles help prevent the vein from collapsing.
Temperature and timing are equally important. Blood chemistry changes the longer a sample sits in contact with blood cells. Glucose and oxygen levels drop while other substances like potassium and phosphorus leak out of cells into the surrounding fluid. Separating the serum within two hours of collection minimizes these shifts. Cooling the sample slows cellular metabolism but introduces its own distortions, so prompt processing is preferred over refrigeration alone.
Every tube is labeled with at least two patient identifiers, typically the patient’s name plus either a medical record number or date of birth. The barcode is verified or attached in the patient’s presence to prevent mix-ups.
Environmental Sampling
When agencies like the EPA collect soil or water to test for contamination, the sampling equipment itself can introduce errors if it’s made of the wrong material. For volatile organic compounds, samples go into specialized containers or pre-prepared vials designed to prevent the chemicals from evaporating before analysis. For persistent chemical contaminants like PFAS (the “forever chemicals” found in many industrial sites), sampling tools must be made of stainless steel, high-density polyethylene, polypropylene, or silicone, because other materials can leach or absorb the very substances being tested for. Even the liner inside a soil sampling tube matters: options include brass, stainless steel, and several types of plastic, each chosen to avoid interfering with the target analysis.
Chain of Custody: Tracking Every Hand
No matter how carefully evidence is collected, it loses its value if there’s a gap in the record of who handled it and when. Chain of custody is the documentation trail that follows every item from the moment it’s collected to the moment it’s presented in court, submitted to a lab, or used in a report.
The National Institute of Justice describes the chain of custody as requiring every person who touches an item to sign for it. A typical custody log records the geographic location where the item was found (often with a photograph), how it was preserved and packaged, and a running list of every person who handled it along with dates and times. Containers must bear complete identification tags and labels. As the item passes from collector to transporter to lab technician to storage, each handoff generates a receipt. A single missing signature can give a defense attorney grounds to challenge the evidence.
What Makes Evidence Admissible
Collection methods directly affect whether evidence holds up in court. Under the Daubert standard, which federal courts and many state courts use, a judge evaluates scientific evidence based on five factors: whether the technique has been tested, whether it’s been published and peer-reviewed, its known error rate, whether there are standards controlling how it’s performed, and whether it’s generally accepted within the relevant scientific community. Evidence collected using untested methods, improperly calibrated equipment, or broken chain-of-custody protocols can be excluded entirely.
In clinical research, a parallel standard exists. Electronic data capture systems used in clinical trials must comply with federal regulations that define when electronic records are considered as trustworthy as paper. These systems require audit trails that log every change, electronic signatures from investigators certifying the data’s accuracy, and validation testing to confirm the software works as intended. Investigators enter patient data directly into the system, which flags inconsistencies in real time rather than waiting weeks for a paper form to reach a data management team.