The object is to find out who can be suspected of the crime and whether or not there is sufficient evidence to initiate an action. The prosecutor leads the preliminary investigation from the point when a certain individual can be reasonably suspected of having committed the offence. In the case of less serious offences, the police conduct the preliminary investigations from beginning to end.
As the person in charge of leading the investigation, the prosecutor is responsible for ensuring that the crime is investigated in the best way possible. The investigations are conducted by the police on the instructions of the prosecutor.
The prosecutor follows the investigations on a continuous basis and constantly determines which investigation measures and decisions are necessary. If the investigation concerns a serious and complicated crime, the prosecutor will often take part directly in the investigation in connection, for instance, with reconstructions of the crime or with important interrogations.
Precisely how a preliminary investigation is conducted in detail depends, of course, on the type of crime being investigated. Following a crime of violence, the police may conduct a crime scene investigation and question victims, witnesses and suspects.
The leader of the preliminary investigation may decide to introduce coercive measures such as cordoning off the crime scene, searching premises or confiscating evidence.
It is often the case that the Swedish National Forensic Centre (NFC) or the National Board of Forensic Medicine (RMV) are consulted in order to judge technical, chemical or medicinal questions that are of importance to the investigation. NFC’s analyses of firearms, narcotics and DNA traces are important features of criminal investigation work, and may sometimes be decisive for the results of the preliminary investigations.
Each year the RMV analyses a large number of blood samples in order to investigate the influence of drugs in connection with a suspected narcotics offence or drunken driving. When investigating a crime of violence, the RMV’s forensic report may provide the answer to what kind of violence may have given rise to the victim’s injuries.
Once the preliminary investigation has been concluded, the prosecutor judges whether or not there is sufficient evidence to bring a court case against the suspect.
The length of time it takes to complete a preliminary investigation depends to a large extent on what it concerns – a preliminary investigation into a case of drunken driving can often be completed over the course of half a day, whereas it may take several years to investigate a serious financial crime.