Organization

The detective branch in most large police agencies is organized into several squads and departments, each of which specializes in investigation into a particular type of crime or a particular type of undercover operation, which may include: homicide, robbery, spot burglary, auto theft, organized crimes, missing persons, juvenile crime, fraud, narcotics, vice, criminal intelligence, aggravated assault/battery, sexual assault, computer crime, domestic violence, surveillance, and arson, among others.

In police departments of the United States, a regular detective typically holds the rank of "Detective". The rank structure of the officers who supervise them (who may or may not be detectives themselves) varies considerably by department. In some Commonwealth police forces, detectives have equivalent ranks to uniformed officers but with the word "Detective" prepended to it (e.g. "Detective Constable").

Private detectives

In some countries, courts and judicial processes have yet to recognize the practice of private detectives. In Portugal, presented proof loses significance when private detectives collect it. Even under these circumstances, the practice is in demand and governed by a code of conduct.^[4]

Citizen detectives

A citizen detective, also known as an amateur detective, is an individual who devotes his or her time and expertise to aid in the solving of crime, without compensation or expectation of reward.^[5] Citizen detectives are private citizens that have no real professional relationship with law enforcement and lack any rational-legal authority whatsoever. The reasons why a private citizen might try to solve a crime vary from trying to ensure justice for a friend or relative, a strong dislike for crime and support for law and order, or just recreational enjoyment.

As with other kinds of detectives, citizen detectives try to solve crimes in multiple ways such as searching a crime scene, interviewing and/or interrogating suspects and witnesses, doing surveillance on persons of interest, collecting evidence, acting as sources for local news, giving anonymous tips to the police and at times even making citizens’ arrests while a crime is being committed. Citizen detectives can also help law enforcement by becoming witnesses for prosecutors, participating in local neighborhood watch groups, acting as citizen observers for law enforcement, or even aiding the police in searching for and arresting suspects as a posse. However, there have been cases of citizen detectives unintentionally compromising investigations if they lack real crime solving skills or even committing acts against suspected criminals that could be deemed vigilantism in nature.

Street work

Edward Bonney, an American bounty hunter and amateur detective from Iowa who in 1845 infiltrated the "Banditti of the Prairie", wrote the 1850 book The Banditti of the Prairies: or, The murderer's doom, a tale of Mississippi Valley and the Far West; woodcut from 1850.

Detectives have a wide variety of techniques available in conducting investigations. However, the majority of cases are solved by the interrogation of suspects and the interviewing of witnesses,^[8] which takes time. Detectives may also use their network of informants, which they have built up over the years. Informants often have connections with people a detective would not be able to approach formally. Evidence collection and preservation can also help in identifying a potential suspect(s).

Criminal investigation

Detectives of the West Midlands Police in the United Kingdom surrounded by electronic devices seized as evidence

Investigation of criminal activity is conducted by the police. Criminal activity can relate to road use such as speeding, drunk driving, or to matters such as theft, drug distribution, assault, fraud, etc. When the police have concluded their investigation, a decision on whether to charge somebody with a criminal offence will (depending on legal jurisdiction) often be made by prosecuting counsel having considered the evidence produced by the police.

In criminal investigations, once a detective has suspects in mind, the next step is to produce evidence that will stand up in a court of law. One way is to obtain a confession from the suspect; usually, this is done by developing rapport and, at times, by seeking information in exchange for potential perks available through the attorney's office, such as entering for a lesser sentence in exchange for usable information. In some countries, detectives may lie, mislead and psychologically pressure a suspect into an admission or confession as long as they do this within procedural boundaries and without the threat of violence or promises outside their control. This is not permitted in England and Wales, where interview of suspects and witnesses is governed by the Police and Criminal Evidence Act 1984 and its extensive Codes of Practice.

Forensic evidence

Physical forensic evidence in an investigation may provide leads to closing a case. Forensic science (often shortened to forensics) is the application of a broad spectrum of sciences to answer questions of interest to the legal system. This may be in relation to a crime or to a civil action. Many major police stations in a city, county, or state, maintain their own forensic laboratories while others contract out the services.

Records investigation

Detectives may use public and private records to provide background information on a subject. Police detectives can search through files of fingerprint records. Police maintain records of people who have committed felonies and some misdemeanors. Detectives may search through records of criminal arrests and convictions, photographs or "mug shots" of persons arrested, hotel registration information, credit reports, answering machine messages, phone conversations, surveillance camera footage, and technology used for communication.

United Kingdom

Before 2017, prospective British police detectives must have completed at least two years as a uniformed officer before applying to join the Criminal Investigation Department. Since 2017, applicants from outside the police force have been able to apply directly for positions as trainee detectives.^[9] UK Police must also pass the National Investigators' Examination in order to progress to subsequent stages of the Initial Crime Investigators Development Programme in order to qualify as a detective.^[10]

United States

Detective escorting gangster Meyer Lansky to the 54th Street police station in New York City in 1958

Before becoming a police detective, one must attend a law enforcement academy, which provides the officer with a foundation of education with 16 to 24 college units. After graduation from the law enforcement academy, the officer undergoes job training with a field training officer for a period specified by the law enforcement agency and continues to work while on a probationary period, ranging from one to two years.

During the probationary period, the officer is assigned to look for evidence. During this time, the officer is supervised and mentored by a sergeant with years of experience. Some police officers go to a two-year or four-year college or university to get a degree in criminal justice or the management of criminal justice. You can get a concentration or a certificate in a specialized field of criminal investigation at some colleges.

Through years of on-the-job training or college education, officers may participate in a competitive examination, testing their knowledge, skills and abilities regarding criminal investigation, criminal procedure, interview and interrogation, search and seizure, collection and preservation of evidence, investigative report writing, criminal law, court procedure, and providing testimony in court. Competitive examinations are conducted by selected senior law enforcement officials. Following testing, a list of results is provided by the department. At the department's discretion, some or all of the officers on the list are promoted to the rank of detective. Some departments have classes of detectives which increase the detective's rank after successful experience.

Private investigators are licensed by the state in which they work (some states do not require licensing, but most do). In addition to the state examination, applicants testing for a private investigation license must also meet stringent requirements, which include college education, a range of two to four years of full-time investigation experience and the successful adjudication of a criminal and civil background check conducted by state investigators. Private investigators are licensed to conduct civil and criminal investigations in the state in which they are licensed, and are protected by statutes of that state. In states requiring licensure, statutes make it unlawful for any person to conduct a criminal investigation without a license, unless exempted by the statute (i.e., law enforcement officers or agents, attorneys, paralegals, claims adjusters).