Investigations

What is a criminal investigation?

A criminal investigation is a legally authorised process for establishing whether an offence occurred and gathering evidence that can be tested in court. This guide explains its structure, safeguards, and known failure modes.

Fact-checkedSafety review clearedLast updated 23 July 2026Last reviewed 23 July 2026

In short

A criminal investigation is a legally authorised process for establishing whether a criminal offence has occurred, who was involved, and whether there is evidence capable of being tested in court.

It is better understood as a legal procedure than as a search. Each significant step requires a basis in law, is recorded, and is capable of being examined afterwards by people who were not present — a prosecutor, a court, a complaints body, or an appellate judge.

Why it exists

A court cannot decide a criminal case on assertion. Someone has to establish what can be shown, to a standard that will survive challenge, and to do so through means the law permits.

The formality exists because of what is at stake in both directions. Evidence gathered without authority may be excluded, ending a case against someone who did commit an offence. Evidence gathered carelessly may be unreliable, contributing to the conviction of someone who did not. The procedural structure is an attempt to reduce both errors at once.

How it works

Investigations vary in sequence, but the same structural elements recur across systems.

Authorisation: Intrusive steps generally require approval from someone outside the investigating team — a supervisor, a prosecutor, or a judge — at a threshold defined in law. That external approval is the principal constraint on the process.
Preservation: Securing material before it is lost or altered. What is not preserved early frequently cannot be recovered later.
Recording: Documenting what was found, where, by whom, and when. The record is what allows a finding to be verified rather than accepted.
Chain of custody: An unbroken account of who held material and when. A gap does not merely weaken evidence; it can remove it from the case entirely.
Interviews under safeguards: Questioning conducted with recording, access to legal advice, and protections for vulnerable interviewees. These requirements exist because of documented cases of unreliable admissions.
Testing the theory: Many systems place an explicit duty to pursue lines of enquiry that point away from a suspect, because confirmation bias is a documented failure mode rather than a hypothetical one.
Disclosure: Material that undermines the case or assists the defence must generally be provided to the defence, whether or not the prosecution relies on it.

Fair-trial guarantees, including adequate time and facilities to prepare a defence, constrain what an investigation must produce and when, in states party to the International Covenant on Civil and Political Rights.

Common misconceptions

Widely held beliefs that do not match how the system actually operates.

Common belief: Investigators can pursue any line of enquiry they judge useful.
In practice: Intrusive steps require legal authority and generally external authorisation. An investigator who wants to search premises, obtain communications data, or hold someone for questioning must satisfy a defined threshold and, in most cases, convince someone outside the team.
Common belief: A confession settles a case.
In practice: Admissions obtained without required safeguards may be inadmissible, and unreliable confessions are a documented cause of wrongful conviction. Many systems require supporting evidence and treat an uncorroborated admission with caution.
Common belief: Investigations are fast when investigators are competent.
In practice: Much of the elapsed time is authorisation, forensic examination queues, disclosure review, and coordination between agencies. These are features of a process designed to be reviewable, not symptoms of a process being performed badly.
Common belief: If the investigation is thorough, the right outcome follows.
In practice: Thoroughness is necessary and not sufficient. Documented miscarriages of justice have involved substantial investigative effort directed by an early theory that was not tested — which is why the duty to pursue exculpatory lines is a formal requirement rather than a matter of professional instinct.

How this varies between jurisdictions

A required section on every guide. Arrangements differ between countries, and we say how.

Who leads differs fundamentally: police-led investigations with prosecutorial oversight, prosecutor-led investigations, or investigations supervised by an investigating judge. Almost everything else follows from this difference.
Judicial authorisation thresholds for searches, communications data, and detention vary, as do the maximum periods a person may be held before being charged or brought before a court.
The scope of disclosure obligations, and the point at which they arise, differ substantially between systems.
Rules on the consequences of unlawfully obtained evidence range from automatic exclusion, through judicial discretion, to admission with the breach going to weight.
Post-conviction review of possible miscarriages is handled by a dedicated body in some countries and by the appellate courts or a ministry in others.

United Nations standards and norms in crime prevention and criminal justice address several of these areas, though implementation is a matter for each state.

Rights and accountability

The rights that apply during an investigation are mostly constraints on investigators, and each generates a record that makes later review possible.

To be informed of the reason for arrest or detention, in a language the person understands.
To have detention reviewed by a court within a defined period.
To access legal advice, with the point at which this arises differing between systems.
To have interviews recorded, and to have safeguards where the person is a child or otherwise vulnerable.
To disclosure of material relevant to the defence.
To complain to a body outside the investigating organisation.

Where to go next

This guide explains what an investigation is. Who is legally competent to run one, and how systems differ, is covered by who investigates crime, police and prosecutor investigation and the investigating judge. Also related: what a prosecutor does, what forensic science is, and why justice systems need oversight. For what an investigative act touches rather than who may perform it, see what privacy protects in law, searching a home and intercepting communications. For the statutory authority beneath those constitutional rules, see legal authority and technical capability.

Who is legally responsible for investigating a crime?
“The police” is an incomplete answer almost everywhere. Legal responsibility for an investigation may rest with police, with a prosecutor, or with a judge — and the body that performs the work is often not the body that owns it.
What is the difference between a police investigation and a prosecutor-led investigation?
Systems differ on who owns an investigation, who performs it, and who may direct whom. These are three separate questions, and the answers do not track each other — the body doing the work is frequently not the body responsible for it.
What does a prosecutor do?
A prosecutor decides whether a criminal case should be brought, on what charges, and presents it in court. This guide explains the tests applied, the duties owed, and how the role differs between systems.
What is forensic science?
Forensic science is the application of scientific methods to questions arising in legal proceedings. This guide explains what it can establish, where its limits lie, and how its reliability is assessed.
Why do societies need law enforcement?
Law enforcement exists so that rules apply to everyone rather than to whoever can be compelled. This guide explains the function, the conditions that make it legitimate, and what it cannot do.

Sources

International Covenant on Civil and Political Rights (United Nations Treaty Series, vol. 999, No. I-14668)
United Nations · International organisation · 1966 · link verified 2026-07-23
Authentic treaty text as registered in the UN Treaty Series; registered ex officio 23 March 1976. Supports the fair-trial guarantees of Article 14, including the right to a competent, independent and impartial tribunal established by law, and the presumption of innocence (Art. 14(2)). Binding only on states party to it; does not support claims about states that have not ratified, nor about how any individual state applies it. WAVE 19 ADDITION, read from the same authentic treaty text in the UN Treaty Series PDF, text-extracted locally. Supports Article 14(5) in full: "Everyone convicted of a crime shall have the right to his conviction and sentence being reviewed by a higher tribunal according to law." The qualifier "according to law" is part of the provision and is load-bearing: the right is to review as the law of the state provides, not to appeal without limit or by any chosen route. Supports Article 14(6): where a person has by a FINAL decision been convicted of a criminal offence and the conviction has subsequently been reversed or the person pardoned on the ground that a new or newly discovered fact shows conclusively that there has been a miscarriage of justice, the person who has suffered punishment as a result shall be compensated according to law, unless it is proved that the non-disclosure of the unknown fact in time is wholly or partly attributable to that person. Supports Article 14(3)(g), that no one shall be compelled to testify against himself or to confess guilt, and Article 14(4), that the procedure for juvenile persons shall take account of their age and the desirability of promoting their rehabilitation. WAVE 20 ADDITION. PROVENANCE, stated precisely because this record's url is a different copy: the Article 4 material below was read on 2026-08-26 from OHCHR's official PDF of the Covenant at https://www.ohchr.org/sites/default/files/ccpr.pdf. The United Nations Treaty Series copy at this record's url timed out repeatedly this session and was not re-read, so nothing in this addition rests on it. Article 4 is the derogation clause, and its four cumulative conditions are the point. Supports ICCPR Art. 4(1): “1. In time of public emergency which threatens the life of the nation and the existence of which is officially proclaimed, the States Parties to the present Covenant may take measures derogating from their obligations under the present Covenant to the extent strictly required by the exigencies of the situation, provided that such measures are not inconsistent with their other obligations under international law and do not involve discrimination solely on the ground of race, colour, sex, language, religion or social origin.”. Supports ICCPR Art. 4(2), read with Arts. 6, 7, 8, 11, 15, 16 and 18 as printed in the same document: “2. No derogation from articles 6, 7, 8 (paragraphs 1 and 2), 11, 15, 16 and 18 may be made under this provision.”. Supports ICCPR Art. 4(3): “3. Any State Party to the present Covenant availing itself of the right of derogation shall immediately inform the other States Parties to the present Covenant, through the intermediary of the Secretary-General of the United Nations, of the provisions from which it has derogated and of the reasons by which it was actuated. A further communication shall be made, through the same intermediary, on the date on which it terminates such derogation.”. The Covenant binds States Parties to it. It is NOT evidence of any particular state's domestic law, and no country claim on this platform rests on it. It designates no organ to proclaim an emergency, assigns no role to any national legislature, and creates no civil-protection institution. The Human Rights Committee's interpretation of this article is cited separately and attributed to the Committee.WAVE 21 ADDITION. Read from the same United Nations Treaty Series PDF, text-extracted and searched. Supports Art. 9(1)-(4), and specifically that the Covenant polices ARBITRARINESS rather than closing a list of grounds: “Everyone has the right to liberty and security of person. No one shall be subjected to arbitrary arrest or detention. No one shall be deprived of his liberty except on such grounds and in accordance with such procedure as are established by law”; with 9(2) (informed at the time of arrest of the reasons and promptly of any charges), 9(3) (brought promptly before a judge or other officer authorised by law to exercise judicial power; it shall not be the general rule that persons awaiting trial shall be detained in custody) and 9(4) (entitlement to take proceedings before a court which decides without delay on the lawfulness of the detention). Supports Art. 14(1): “All persons shall be equal before the courts and tribunals”, with the right to a fair and public hearing by a competent, independent and impartial tribunal established by law, the closed grounds on which press and public may be excluded, and the rule that judgment shall be made public except where the interest of juvenile persons otherwise requires or the proceedings concern matrimonial disputes or the guardianship of children. Supports Art. 14(3)(e) (“To examine, or have examined, the witnesses against him and to obtain the attendance and examination of witnesses on his behalf under the same conditions as witnesses against him”), 14(3)(f) (free assistance of an interpreter) and 14(3)(g) (“Not to be compelled to testify against himself or to confess guilt” — note that the Covenant adds the confession limb, which the European Convention text does not contain). Supports Art. 14(5). VERIFIED NEGATIVES from exhaustive search of the Covenant text: “proportion” occurs zero times, “equality of arms” zero times, and “remain silent” zero times. This is treaty text binding states party to it; it is not evidence of any state's domestic law, and the corpus invariant forbidding a treaty from standing in for a country applies to it.
Crime Prevention and Criminal Justice
United Nations Office on Drugs and Crime · International organisation · link verified 2026-07-23
Supports the existence of the UN crime prevention and criminal justice programme and its standards-and-norms work across policing, courts, and prisons. Does not support any country-level claim.