The first officer at a scene decides how much of it survives. Almost everything in the published guidance for that first hour is about restraint — what not to touch, not to move, not to let happen — because a scene can only be disturbed once.
The order the guidance gives, and it is an order rather than a list:
- Your own safety first, then everybody else's. A scene that injures a second person has stopped being an investigation and become an incident. Look for what is still dangerous before looking for what is evidential: traffic, fire, structural collapse, a live electrical supply, a weapon unaccounted for, a person still present.
- The living come before the evidence. Anybody who needs medical help gets it, and the scene is disturbed to whatever extent that requires. This is not a compromise between two goods; it is settled.
- Establish the boundary, and make it wider than you think. Guidance is consistent that first cordons are drawn too tight. A boundary can be brought in later; it cannot be pushed back out once people have walked through the space.
- Control who enters. A single route in and out, and a written record of every person who used it, with the time. The record matters as much as the cordon: it is what later distinguishes a trace left by an offender from one left by a colleague.
- Write down what you found, before it changes. Doors open or shut, lights on or off, weather, smells, what was moved to reach a casualty and by whom. Anything altered for a good reason has to be recorded as altered, or the alteration becomes an unexplained discrepancy months later.
Source. National Institute of Justice, Crime Scene Investigation: A Guide for Law Enforcement (NCJ 178280), https://www.ojp.gov/pdffiles1/nij/178280.pdf — and UNODC, Crime scene and physical evidence awareness for non-forensic personnel, https://www.unodc.org/documents/scientific/Crime_scene_awareness__Ebook.pdf
What this is not. The association is not a police force. Procedure at a real scene is set by the law of that country and by the instructions of the service the officer belongs to, and where those differ from this page, they govern and this page does not.