The 7 S’s of crime scene investigation, in order, are: secure the scene, separate the witnesses, scan the scene, see the scene, sketch the scene, search for evidence, and secure and collect evidence. The sequence is deliberate. Each step preserves something the next step depends on, and a step performed out of order, or skipped, can contaminate physical evidence, distort witness accounts, or give a defense attorney grounds to have evidence thrown out at trial.
1. Secure the Scene
Everything begins with locking down the area. The first officer on scene establishes boundaries using barrier tape, vehicles, cones, or existing structures like walls and fences. Those boundaries start at the focal point of the crime and extend outward to cover potential entry and exit paths, any place the victim or evidence may have been moved, and surrounding areas where trace or impression evidence might exist.1National Institute of Justice. Crime Scene Investigation – A Guide for Law Enforcement
Once the perimeter is up, a crime scene entry log tracks every person who enters or leaves, along with the time and reason for their presence. This log matters more than people realize. Defense attorneys routinely challenge evidence by pointing to unaccounted-for individuals who may have contaminated the scene. The fewer people inside the tape, the stronger the evidence holds up later.
Environmental threats also get addressed at this stage. Rain, wind, foot traffic, and sprinkler systems can destroy evidence. Protecting fragile items like tire tracks in mud or bloodstain patterns on pavement sometimes means improvising with tarps or traffic cones before any formal processing begins.1National Institute of Justice. Crime Scene Investigation – A Guide for Law Enforcement
2. Separate the Witnesses
Before witnesses have a chance to compare notes, investigators pull them apart. This isn’t about suspicion. It’s about accuracy. Research consistently shows that witnesses who talk to each other before giving statements are significantly more likely to report details they didn’t personally observe. Memory is malleable, and hearing someone else’s version of events can overwrite a witness’s own recollection without them noticing.
Each witness gives an independent account based solely on what they saw, heard, or experienced. Investigators take these initial statements separately and out of earshot of other witnesses. When physical separation isn’t possible, during a chaotic street incident with dozens of bystanders for example, officers at minimum remind each witness to describe only what they personally observed. People who offer to “support” a witness during their account may themselves be witnesses and shouldn’t be allowed to prompt or speak for someone else.
3. Scan the Scene
Scanning is the slow, deliberate walkthrough before anyone touches anything. The investigator takes in the overall layout, identifies where key evidence sits, and begins forming a mental map of what happened. This overview shapes the entire strategy for what follows: where to photograph first, which areas to prioritize during the search, and what resources the scene will require.
A good scan also identifies secondary scenes. A break-in might have a primary scene inside a home, but the point of entry around back, a getaway vehicle route, or a discarded tool in a neighboring yard are all secondary scenes that need the same level of protection. Experienced investigators look for things that seem out of place: an open window in winter, a chair moved away from a table, a missing item where dust outlines suggest something was removed.
4. See the Scene
“See the scene” is the framework’s term for photography and videography. Before anything is moved, picked up, or closely examined, the entire scene is captured visually. This creates a permanent record of exactly how things looked when investigators arrived.
Photography follows a three-layer approach: overall shots that capture the full scene and its surroundings, mid-range photographs showing how pieces of evidence relate to each other in space, and close-ups of individual items. Close-up evidence photographs include a measurement scale alongside the item so analysts can determine actual dimensions from the image alone.1National Institute of Justice. Crime Scene Investigation – A Guide for Law Enforcement Forensic photo scales allow investigators to reconstruct the dimensional context of the scene later and produce true-to-size reproductions of physical evidence.2National Institute of Standards and Technology. Dimensional Review of Scales for Forensic Photography
Investigators also photograph the crowd outside the perimeter, nearby vehicles, and the victims, suspects, and witnesses themselves. Video is sometimes used as a supplement to still photography, particularly for complex scenes where spatial relationships are hard to convey in static images.1National Institute of Justice. Crime Scene Investigation – A Guide for Law Enforcement
5. Sketch the Scene
Photography captures what things look like. Sketches capture where things are. A crime scene sketch is a measured diagram showing the spatial relationships between evidence, structures, furniture, and other landmarks, information that even high-quality photographs can’t fully convey.
Two versions are created. The rough sketch is drawn on-site before any evidence is moved. It includes all measurements, evidence locations, directional indicators, and case identifiers. Rough sketches don’t need to be drawn to scale, but they preserve every measurement and data point recorded at the scene. Nothing gets added or deleted after investigators leave.3Federal Law Enforcement Training Center. Crime Scene Sketch – Student Guide SG-205
The finished drawing comes later. It’s prepared from the rough sketch, drawn to scale, and cleaned up for courtroom presentation. Unlike the rough sketch, the finished version omits the raw measurements to keep the visual uncluttered. Both versions help with reconstruction, refreshing a witness’s memory, and helping a jury understand the physical layout.3Federal Law Enforcement Training Center. Crime Scene Sketch – Student Guide SG-205
6. Search for Evidence
With the scene documented, investigators conduct a systematic physical search. The goal is completeness: making sure no relevant evidence gets overlooked. Different scenes call for different search patterns, and the choice depends on the area’s size, layout, and number of available personnel.
- Strip or line search. Searchers walk in parallel straight lines from one end of the area to the other. Best for large outdoor areas where consistent ground coverage matters.
- Grid search. The same area is searched twice in perpendicular directions, essentially running the strip method in both orientations. The double pass catches items that a single sweep might miss.
- Spiral search. Investigators move in a circular path either inward toward a focal point or outward from it. Useful when searching for a specific object, like a shell casing near a shooting location.
- Zone or quadrant search. The scene is divided into smaller sections, and each section is searched independently. Works well for complex indoor scenes or areas with a lot of physical obstacles.
For interior scenes involving bloodstain patterns or firearm use, some investigators also use an elevation-based approach, working from the floor upward to trace spatter origins and directionality along walls and ceilings.
7. Secure and Collect Evidence
The final S covers the physical recovery of evidence. Every item is handled carefully to prevent contamination, then packaged in a container matched to the evidence type. Biological samples go into breathable containers like paper bags or envelopes that allow residual moisture to evaporate. Sealing wet biological evidence in plastic is one of the fastest ways to destroy it, because trapped moisture accelerates bacterial degradation. Non-biological evidence goes into paper bags, envelopes, or boxes depending on size and fragility.
Each package gets labeled with a unique identifier, the case number, the date and time of collection, and the collector’s name. Every seal is initialed and dated. The level of detail might seem excessive, but a single mislabeled evidence bag can create reasonable doubt at trial.
Digital Devices
Mobile phones, tablets, and computers present a challenge that didn’t exist a generation ago. These devices can be remotely wiped, have data altered, or receive new information after seizure if they remain connected to cellular or Wi-Fi networks. Standard practice is to place wireless-capable devices in a Faraday bag immediately, a shielded enclosure that blocks cellular, Bluetooth, and Wi-Fi signals. When Faraday bags aren’t available, wrapping a device several times in aluminum foil can work as a temporary substitute, though it’s less reliable. Power cables should never be threaded through the opening of a Faraday bag, because the cord can act as an antenna and defeat the shielding.
If the device screen is accessible, enabling airplane mode and manually confirming that Bluetooth and Wi-Fi are turned off adds another layer of protection. A Faraday bag is still recommended afterward, because a device owner can sometimes track and trigger actions through “lost device” features that override airplane mode.
Trace Evidence
Tiny items like hairs, fibers, and paint scrapings require special containment. These are placed into a “druggist fold,” a carefully creased piece of clean paper that creates a sealed pocket, before being placed inside a labeled envelope. Putting hairs or fibers directly into an envelope risks losing them in corners or having them contaminated by adhesive. The druggist fold itself is never sealed with tape, because adhesive residue can compromise the evidence inside.
Chain of Custody After Collection
All of the care taken during collection means nothing if the evidence can’t be accounted for afterward. Chain of custody is the documented trail tracking every person who handles a piece of evidence from the moment it leaves the crime scene until it’s presented in court. Each transfer requires a signature, date, and time. Each storage location must be secure enough that no unauthorized person can access the evidence.4National Institute of Justice. A Chain of Custody – The Typical Checklist
A chain of custody form accompanies each evidence item and records, at minimum, the unique identifier for the item, the collector’s name and agency, the date and time of collection, a description of the item, and every subsequent transfer with signatures. Any gap in this record, a missing signature, an unaccounted-for period, evidence stored in an unsecured location, gives the defense grounds to argue the item may have been tampered with, contaminated, or substituted. At trial, no question should exist about missing items, mishandling, mislabeling, or breaks in the chain that might jeopardize admissibility.4National Institute of Justice. A Chain of Custody – The Typical Checklist
Why the Order Matters Legally
The seven steps aren’t just procedure. Failures at any stage carry legal consequences. The most significant involve the Fourth Amendment, which protects people from unreasonable searches and seizures and requires warrants to be based on probable cause, supported by sworn statements, and specific about the place to be searched and items to be seized.5Library of Congress. U.S. Constitution – Fourth Amendment
Investigators sometimes assume that the seriousness of a crime, particularly a homicide, justifies entering and searching a scene without a warrant. The Supreme Court rejected that reasoning directly. In Mincey v. Arizona, the Court held that there is no “murder scene exception” to the warrant requirement, and that the seriousness of the offense under investigation does not by itself create the kind of emergency that justifies a warrantless search.6Justia. Mincey v. Arizona, 437 U.S. 385 (1978) Warrantless entry is permitted only under recognized exceptions like active emergencies where delay could lead to injury or destruction of evidence, evidence in plain view, or consent from someone with authority over the premises.
When investigators violate the warrant requirement without a valid exception, the exclusionary rule applies. Evidence obtained through an unconstitutional search is suppressed, meaning it cannot be used at trial. The Supreme Court has described this rule as the only effective way to enforce Fourth Amendment protections, because it removes the incentive for law enforcement to disregard the constitutional requirement.7Library of Congress. Amdt4.7.2 Adoption of Exclusionary Rule
Even when the search itself is legal, sloppy execution of any of the seven steps can undermine a prosecution. Contaminated DNA evidence, incomplete documentation, and broken chains of custody have contributed to acquittals and overturned convictions. Incomplete records cast doubt on evidence integrity and can render otherwise powerful forensic evidence inadmissible. The defense doesn’t need to prove tampering occurred. They only need to show it could have.