Arrest and prosecution functions are separated primarily to protect citizens against the arbitrary exercise of police power. Police officers usually make arrests based only on whether they have good reason (probable cause) to believe a crime has been committed. By contrast, prosecutors can file formal charges only if they believe that they can prove a suspect guilty beyond a reasonable doubt.
Prosecutors can also take a broader perspective. They have what is called "prosecutorial discretion." Prosecutors can look at all the circumstances of a case, including the suspect's past criminal record, in deciding whether and what to charge. Prosecutors can file charges on all crimes for which the police arrested a suspect, can file charges that are more or less severe than the charges leveled by the police, or can decide not to file any charges at all. (U.S. v. Batchelder, U.S. Sup. Ct. 1979.)
Typically, prosecutors base their initial charging decisions on the documents sent to them by the arresting police officers (usually called police or arrest reports). The police complete an arrest report soon after they make an arrest and then quickly forward the report to a prosecutor assigned to do case intake. Arrest reports summarize the events leading up to arrests and provide numerous other details, such as dates, time, location, weather conditions, and witnesses' names and addresses.
Arrest reports are almost always one-sided. They recite only what the police claim took place and may include only witness statements that support the police theory. While they are generally not admissible as evidence in a trial, arrest reports can have a major impact in criminal cases.
Not only do arrest reports often determine what charges prosecutors file, but they also may play a key role in how much bail is required, the outcome of preliminary hearings (where hearsay evidence is often admissible), the willingness of the prosecutor to plea bargain, and trial tactics (for instance, the police report can be used to discredit testimony of the police officer who prepared the report).
Most head prosecutors are elected officials. Many of them view their position as a stepping-stone to higher office. Their charging decisions are often, therefore, affected by public opinion or important support groups. For example, a prosecutor may file charges on every shoplifting case, no matter how weak, to curry favor with local store owners who want to get the word out that shoplifters will be prosecuted. For similar reasons, a prosecutor may pursue otherwise weak prostitution charges to avoid alienating powerful civic groups. Deputy or assistant prosecutors may feel that appearing tough will help their careers, either within the prosecutor's office or later if they want to become judges.
Experienced defense attorneys understand that prosecutors must sometimes be seen as taking a strong stand publicly, even though they may be willing to respond to weaknesses in individual cases at a later stage of the process. This is one of the reasons why practically every criminal defendant will benefit from the help of an experienced, local criminal defense attorney: Only those professionals know where the pressure points are and how to work around them (or with them).
This article was excerpted from The Criminal Law Handbook, by Paul Bergman, J.D., and Sara J. Berman, J.D.