Crime scene fingerprints are collected in a set sequence: secure the area, locate the prints, make invisible ones visible with powders or chemicals, photograph each print with a scale, then physically recover it by tape lift, casting, or seizing the object it sits on. Every print is labeled and entered into a chain of custody log the moment it leaves the scene. The specific technique an investigator reaches for depends on two things: whether the print is visible, and what surface it’s on.
The Three Types of Prints Investigators Encounter
What an examiner does first depends on which kind of print is in front of them.
- Latent prints are invisible. They come from the natural oils and sweat that transfer from skin to a surface on contact, and they make up the majority of prints recovered from crime scenes. They have to be developed before anyone can see them.
- Patent prints are visible without any enhancement. A finger coated in blood, ink, grease, or dirt leaves a clear impression on contact. They still need careful photography, and sometimes chemical treatment to sharpen the ridge detail.
- Plastic prints are three-dimensional impressions pressed into a soft material like wax, putty, wet paint, or soap. The surface itself holds the ridge pattern in relief.
What makes any of these useful for identification is the arrangement of small features along the friction ridges: endings where a ridge stops, bifurcations where one ridge splits into two, short ridges, enclosed ridges, and dots. An average fingerprint contains roughly 150 of these characteristics. Sweat pore positions and the shape of ridge edges give examiners finer detail for comparison.
Why Speed Matters
Prints don’t last forever. How long a latent print stays recoverable depends on the surface, what the finger deposited, and the environment. Research tracking latent print degradation over six months found that prints left by oily skin on glass were still identifiable at the end of the study, while prints from watery sweat on plastic degraded quickly and often became useless. Smooth, nonporous surfaces like glass and metal preserve prints far longer than porous or textured ones. Heat, humidity, and wind speed up degradation on any surface. That is one reason fingerprint recovery is placed early in the scene processing sequence.
Securing the Scene Before Any Collection Begins
The first officer on scene establishes a perimeter and controls entry. Every additional person walking through a scene risks destroying, moving, or contaminating prints. Standard evidence-collection protocols restrict entry to authorized personnel and require officers to minimize disturbance and avoid introducing foreign materials.1Florida State University Police Department. Collection and Preservation of Evidence
With the perimeter set, investigators walk the scene and identify surfaces most likely to hold prints. Doorknobs, light switches, window glass, weapon grips, countertops, and any object that looks moved or handled get priority. That initial survey decides which visualization techniques come out of the kit and in what order.
Making Latent Prints Visible
Because latent prints can’t be seen, the core work at most scenes is developing them into something an examiner can photograph and collect. The technique tracks the surface.
Powder Dusting on Nonporous Surfaces
On glass, metal, polished wood, and hard plastic, investigators apply fine powder with a brush. The powder clings to the moisture and oily residue in the print and reveals the ridge pattern. Black powder works on light surfaces, white or gray on dark ones, and fluorescent powders show up under ultraviolet light against multicolored backgrounds. Dusting is fast and effective, which is why it remains the most common field technique for nonporous items.
Chemical Reagents on Porous Surfaces
Paper, cardboard, and unfinished wood absorb the residue that powder would otherwise cling to, so dusting rarely works. Examiners use chemical reagents that react with components of the fingerprint deposit itself. Ninhydrin is the workhorse: it reacts with amino acids in sweat residue to produce a purple-blue stain that reveals the ridge pattern. The reaction can take hours or days at room temperature and speeds up with heat and humidity. DFO is another porous-surface reagent, often applied before ninhydrin because it fluoresces under specific wavelengths and can reveal prints ninhydrin misses.
Bloodstained evidence calls for a different tool. Amido black is a protein-staining dye used to enhance fingerprints deposited in or contaminated by blood, producing dark blue ridge detail against the stained background.
Cyanoacrylate (Superglue) Fuming
When a nonporous item needs a more permanent result than powder produces, investigators use cyanoacrylate fuming. The item goes into a sealed chamber with a small amount of superglue, which is heated to release fumes. Those fumes react with fingerprint residue and polymerize into a hard, white deposit that outlines the ridge detail. The result is stable and won’t smudge during further handling. The technique was first developed by the Japanese National Police Agency in 1978 and has since become standard worldwide.2National Center for Biotechnology Information. Cyanoacrylate Fuming Method for Detection of Latent Fingermarks
Forensic Light Sources
Forensic light sources use specific wavelengths, from ultraviolet through the visible spectrum, to make latent prints fluoresce or stand out against a surface. A 365nm UV light can reveal prints that are nearly invisible under normal lighting. Light examination is especially useful as a preliminary scan before powders or chemicals go on, because it doesn’t alter the print. The same tools also enhance prints already treated with fluorescent powders or certain chemical reagents, pulling out ridge detail that would otherwise be too faint to photograph.3Office of Justice Programs. Shining Light on Fingerprints to Fibers to Fluids: Alternative Light Sources Reveal Many Types of Evidence
Capturing the Print Once It’s Visible
Photography Comes First
Every print gets photographed before anything else happens. The photograph captures the print as the examiner first encountered it, before lifting tape touches it or casting material covers it. Investigators shoot the overall location, then move in with close-up lenses and a measurement scale placed beside the print. Specialized lighting and angles bring out ridge detail. The photographs are a permanent record even if the physical lift is later damaged, and they’re the version typically used for comparison and measurement.4National Institute of Standards and Technology. Standard Guide for Crime Scene Photography
Lifting Powder-Developed Prints
After the photograph, the examiner presses clear adhesive tape over the developed print, smooths it down to pick up the powder pattern, and carefully peels it away. The tape goes onto a backing card in a contrasting color, creating a portable record of the print. The lifted print is placed in a labeled evidence envelope.
Casting Plastic Prints
Three-dimensional impressions in soft materials can’t be lifted with tape. Examiners pour casting material, typically dental stone or silicone, into the impression and let it set. The result is a detailed replica of the ridge pattern. When the object bearing the print is small enough to transport, investigators often skip the cast and collect the whole item for lab processing.
When Fingerprints and DNA Compete on the Same Surface
Latent prints often contain enough skin cells to yield a DNA profile, and that creates a real tension at the scene: processing for prints can degrade the DNA, and swabbing for DNA can destroy the print. Treating a print with DFO or ninhydrin significantly reduces the amount of DNA later recoverable from that same area. Best practice is generally to collect DNA swabs from areas that may yield a profile before applying fingerprint chemicals to those spots. Briefly touched items tend to be better fingerprint candidates, while items with prolonged skin contact often favor DNA collection.
Preventing Contamination
Contamination is the fastest way to make fingerprint evidence worthless. A single procedural lapse can introduce foreign material, transfer someone else’s DNA onto the evidence, or hand a defense attorney grounds to challenge everything recovered from the scene.
The National Institute of Justice recommends that investigators wear double gloves and change the outer pair frequently, use disposable instruments or thoroughly clean reusable ones between samples, air-dry wet evidence before packaging, and use paper bags rather than plastic to prevent moisture buildup and mold.5National Institute of Justice. Tips to Protect Crime Scene Evidence
Fingerprint brushes deserve their own attention. Reusing a brush across multiple items or scenes has been shown to accumulate and redeposit DNA, potentially contaminating later evidence. The recommended practice is a separate disposable brush for each item and fresh powder from a clean container rather than repeated dips into one supply. The superglue fuming chamber carries the same risk: DNA from a previous case can transfer to new evidence if the chamber isn’t cleaned or filtered between uses.
Labeling and Chain of Custody
Every recovered print is labeled with the case number, collection date and time, its exact location at the scene, and the initials of the person who collected it. Crime scene sketches and written notes supplement the photographs to record spatial context that a close-up alone can’t convey.
Lifted prints on backing cards, casts, and collected objects are individually packaged in materials that protect against environmental damage and accidental contact. From the moment evidence is collected, a chain of custody log tracks every person who handles it, every transfer between agencies, and every period of storage. The purpose is to prevent substitution, tampering, contamination, or misidentification. Each person who takes possession must be identified, and every period of custody must be accounted for.6National Institute of Justice. Law 101: Legal Guide for the Forensic Expert – Chain of Custody
A broken chain of custody can result in the evidence being excluded from trial entirely or given reduced weight by the jury. Sloppy documentation at the scene creates consequences months or years later in a courtroom, which is why the collection stage is where good fingerprint cases are won or lost.