After an arrest for a criminal offense, the arrest report is sent to a prosecutor, whose job it is to initiate and prosecute criminal cases. The report summarizes the events leading up to arrest and provides numerous other details, such as dates, time, location, and weather conditions of the crime, and witnesses' names and addresses, if that information is available. From there, the prosecutor will either:
- decide that the case should be charged (as a felony or a misdemeanor), and file a complaint with the trial court.
- decide that the case should be charged as a felony and bring evidence before citizens serving as grand jurors, who will decide what charges, if any, to file
- decide that the matter should not be pursued.
Prosecutors can file charges on all crimes for which the police arrested a suspect., or they can file charges that are more or less severe than the charges leveled by the police.
For suspects who are in custody, laws usually require prosecutors to file charges, if at all, within 72 hours of the arrest, though some jurisdictions require prosecutors to charge a suspect even sooner. However, prosecutors' initial charges are subject to change.
A prosecutor's decision to file charges may be influenced by factors beyond the specifics of the incident described in the police report. Some prosecution offices adopt policies on certain types of crimes, often in response to community pressure, and these may determine the prosecutor's approach in any given case. Furthermore, prosecutors may also be influenced by their own political ambitions. Most prosecutors are elected officials and many of them view their position as a stepping-stone to higher office. Consequently, their decisions on charges are often affected by public opinion or important support groups. Finally, some decisions are influenced by the prosecutor's sense of what justice requires in the case. Prosecutors are supposed to both enforce the law and "do justice." This means that occasionally a prosecutor decides not to prosecute a case, or files less severe charges, because the interests of justice require it, even if the facts of the case might support a conviction. For example, if an otherwise law-abiding person makes a one-time, foolish mistake, a prosecutor may decide that it would not serve any purpose to spend time and money prosecuting this person, especially when the chances that the person will re-offend are virtually none.
If a felony is involved, prosecutors sometimes leave it to grand juries to decide whether charges should be filed. The grand jurors listen to evidence and decide whether charges should be brought against an individual. However, unlike petit juries, which only sit on one case, grand juries involve a time commitment that typically lasts between 6 and 18 months. The grand jurors may address many cases in the course of their service. In addition, these crucial differences exist:
- Petit jurors decide whether defendants are guilty. Grand juries decide whether there is enough evidence to warrant a trial.
- Grand juries meet in secret proceedings. Petit juries serve during public trials.
- Grand juries have 15-23 people. By contrast, a petit jury usually consists of between 6 and 12 people.
- Petit juries generally have to be unanimous to convict a defendant. Grand juries need not be unanimous to indict. In the federal system, for example, an indictment may be returned if 12 or more jurors agree to indict.
When a prosecutor brings a case to a grand jury, he presents the jurors with a the charges and introduces evidence, usually the minimum necessary, in the prosecutor's opinion, to secure an indictment. The proceedings are secret; it is standard practice to call witnesses to testify against the suspect without the suspect or the suspect's lawyer present. However, indicted suspects can sometimes later obtain transcripts of grand jury proceedings and this is a reason why prosecutors like to keep the evidence to the minimum.
Although the prosecutor can also call the suspect as a witness, this is not typically done. When suspects are called, they often refuse to testify by invoking their privilege against self-incrimination under the Fifth Amendment to the U.S. Constitution.
If the grand jury decides to indict, it returns what is called a "true bill." If not, the grand jury returns a "no-bill." But even if the grand jury returns a no-bill, the prosecutor may eventually file charges against a suspect. Prosecutors can return to the same grand jury with more evidence, present the same evidence to a second grand jury, or, in jurisdictions that give prosecutors a choice, bypass the grand jury altogether and file a criminal complaint.
If the prosecutor decides to file a complaint rather than present the case to a grand jury, and the case is a felony, the defendant is entitled to a preliminary hearing at which the prosecutor must show that the state has enough evidence of the crime to warrant a trial. However, if the case proceeds by grand jury indictment, no preliminary hearing need be held. For this reason many prosecutors choose the grand jury indictment process because they don't have to reveal as much evidence before the trial.