To gain a conviction in countries where the rule of law is firmly rooted, it is essential that the investigating agency gather sufficient legally admissible evidence to convince the judge or jury that the suspect is guilty. Police departments are often reasonably certain that a particular individual is responsible for a crime but may remain unable to establish guilt by legally admissible evidence. In order to secure the necessary evidence, the police employ a variety of powers and procedures. Because those powers and procedures, if exercised improperly, would enable the police to interfere with the constitutionally protected freedoms of the suspect, they are normally subject to close scrutiny by legislation or by the courts.

One important procedure is the search of a suspect’s person or property. Most common-law jurisdictions allow a search to be carried out only if there is “probable cause for believing” or “reasonable ground for suspecting” that evidence will be found. In some cases a person may be stopped on the street and searched, provided that the police officers identify themselves and state the reasons for the search. In the United States a person stopped on the street may be patted down for a weapon without the police’s having any evidence whatsoever. A search of private premises usually requires a search warrant issued by a magistrate or judge. The law generally permits a search warrant to be issued only if the authorities are satisfied (after hearing evidence under oath) that there is good reason to suspect that the sought-after evidence, which the warrant usually defines specifically, will be found on the premises. The warrant may be subject to time limits and normally permits only one search. In most countries the judge or magistrate who issues the warrant must be informed of the outcome of the search. Materials seized as a result of a search under the authority of a search warrant are usually held by the police for production as exhibits at any subsequent trial.

In the United States any evidence discovered as a result of a search that does not comply with the procedures and standards laid down by the courts and legislative bodies is not admissible in court, even if it may clearly establish the guilt of the accused person. Because it may prevent the conviction of a person who is guilty, this doctrine, known as the exclusionary rule, has given rise to controversy in the United States and has not generally been adopted in other countries. The exclusionary rule has been particularly important in drug cases, where the materials seized (i.e., the drugs themselves) often are the only evidence against the defendant; according to the U.S. Department of Justice, adherence to the rule has resulted in the dismissal of about 1 percent of drug cases. However, since its decision in United States v. Leon (1984), the U.S. Supreme Court has adopted several “pro-prosecution” modifications of the exclusionary rule, including a somewhat limited “good faith” exception for the police. That is, if the police attempted to uphold constitutional requirements for the search but made an honest mistake, then the evidence may be admissible at trial even if some constitutional requirements were not met.

Interrogation and confession

An important aspect in the investigation of offenses is the interrogation of suspects. The aim of the questioning is usually to obtain an admission of guilt by the suspect, which would eliminate the need for a contested trial. Most countries place restrictions on the scope and methods of interrogation in order to ensure that suspects are not coerced into confessions by unacceptable means, though in practice the effectiveness of those restrictions varies greatly. In the United States, for example, suspects must be informed that they have certain rights, including the right to remain silent, to have a lawyer present during the interrogation, and to be provided with the services of a lawyer at the expense of the state if they cannot afford one. The statement of rights that is read to suspects, known as the Miranda warnings, was established in the case of Miranda v. Arizona (1966). Failure to advise a suspect of those and other rights can result in the rejection of a confession as evidence.

In contrast, British law focuses on whether the confession itself was voluntary, rather than on whether proper procedures were followed by the police. With minor exceptions, a person suspected or accused of a criminal offense is not required to answer any question or to give evidence. For many years the English law on confessions consisted of a simple rule prohibiting the introduction at trial of any involuntary statement made by an accused person. That rule was supplemented by more-detailed rules governing the questioning of suspected persons by the police, known as the Judges’ Rules. Principally, the Judges’ Rules obliged the investigating police officer to caution suspects that they were not required to answer any question and that anything they did say might be given in evidence at trial. That caution was required to be stated at the beginning of any period of interrogation and immediately before a suspect began to make a full statement or confession. Failure to provide a caution at the right time or in the right form did not necessarily mean that the statement would be excluded from evidence, but trial judges did have the discretion to exclude the evidence. The operation of the Judges’ Rules was a source of controversy for many years; in the mid-1980s they were reformed by a comprehensive series of provisions. The reforms, which were supplemented by detailed codes of practice, allowed a confession to be admitted into evidence provided that it was not obtained by oppression of the person who made it (e.g., by torture, inhuman or degrading treatment, the use or threat of violence, or excessively prolonged periods of questioning) or as a result of anything said or done that would be likely to render the confession unreliable.

Other countries generally have similar legal requirements, though the actual practices in those countries may be quite different. Russia, for example, has a rule on confessions that is quite similar to the Miranda warnings, while in China a suspect has the legal right to remain silent, there is no legal penalty for the refusal to answer questions, and police are forbidden to obtain confessions through the use of force. However, in practice, police in some countries sometimes use physical force to obtain confessions, and such illegally obtained evidence is not excluded at trial.

For full treatment of trial procedures prior to sentencing, see criminal law and procedural law.

Anglo-American countries