Public posts/forums (open webpages, public subforums, social posts): police can view and collect these directly (screenshots, archives, server scraping). Passive collection of publicly available information is generally lawful. Department of Justice
Private messages/accounts (DMs, private groups, closed forums): investigators normally need a legal process (warrant, subpoena, preservation request) to obtain content from the platform or to compel the platform to preserve data while the court process proceeds. Provider cooperation policies and legal standards vary by country. Meta+1

2) Legal tools & cross-border issues

Law enforcement uses preservation requests, subpoenas, court orders and warrants to get data from platforms; international requests can invoke treaties (MLATs) or laws such as the U.S. CLOUD Act for cross-border disclosures. Jurisdictional complexity can delay or limit access. Department of Justice+1

3) Encryption, “warrant-proof” services and technical limits

End-to-end encryption (E2EE) on messaging apps prevents the provider from reading message content — meaning some services cannot comply with content-preservation or disclosure requests because they do not hold the plaintext. That’s a major practical barrier and a live policy/legal flashpoint in many countries. Federal Bureau of Investigation+1

Investigations of hidden services (Tor, some darknet markets) often require specialised techniques: undercover operations, network analysis, cryptocurrency tracing, or seizing servers that host marketplaces. These are technically hard and often international. National Institute of Justice

When police seize devices or obtain copies from providers, digital forensics teams follow strict procedures (imaging/cloning, chain of custody, documented handling) so evidence is admissible in court. International bodies (Interpol, NIJ, etc.) publish guidelines on proper acquisition and preservation. Interpol+1

Platforms publish law-enforcement guidance and will respond to valid legal process — but they also have transparency rules, retention policies, and lawful-access safeguards. Regulators are increasingly forcing platforms to demonstrate they manage illegal content (recent enforcement actions show governments pushing platforms for more cooperation). Meta+1

Don’t delete or alter the account or content.
Take time-stamped screenshots, export files, or preserve original devices. (Document how you collected them.)
Keep a log of who had access (chain of custody).
If a crime is involved, contact authorities or an attorney before altering anything.
If dealing with encrypted platforms, know that providers may have no readable copy — preserve any device that might contain decrypted data. Interpol+1