Federal investigation in the United States is not one manual but a constitutional framework with published guidance underneath it.
What governs. The Fourth Amendment governs search and seizure; the Attorney General's Guidelines for Domestic FBI Operations govern what the Bureau may open and on what basis; the Domestic Investigations and Operations Guide sets out the internal rules. Evidence handling is measured against standards developed through NIST's Organization of Scientific Area Committees for Forensic Science.
What is worth taking from it. Three things that travel:
- A predicate. An investigation is opened on an articulable basis, and the basis is written down before the work begins rather than assembled afterwards.
- The least intrusive method that will work. The guidelines require it explicitly. It is also good practice anywhere: intrusion that is not needed is intrusion that will be challenged.
- Documented authority. Who authorised what, when, and under which provision. A technique used without a recorded authority is a technique that will not survive a court.
Sources. NIST Organization of Scientific Area Committees, https://www.nist.gov/osac ; NIJ, Crime Scene Investigation: A Guide for Law Enforcement, https://www.ojp.gov/pdffiles1/nij/178280.pdf ; the Attorney General's Guidelines and the DIOG are published by the US Department of Justice and the FBI respectively.
Note on directness. The FBI's own web servers refuse automated requests from this platform, so the Bureau's documents are cited here by name and publisher rather than fetched. A reader should go to the source.