It is the prosecutor that makes the final decision as to whether to formally charge an individual or not, not the police department or other law enforcement agency.
In most state criminal cases, police arrest individuals and bring the case to the District Attorney’s Office. The police only need probable cause to make an arrest, but the District Attorney’s Office must be able to prove each and every element of each crime charged beyond a reasonable doubt for a conviction. In federal systems, the investigative work prior to an arrest is usually more substantial.
Prosecutors have discretion on whether or not to charge an individual. They can consider factors such as the strength of the evidence, the individual’s criminal history, the extent of harm or loss allegedly sustained, witness cooperation, resources needed, and weigh it against the interests of justice. Prosecutors have an ethical obligation to pursue only cases they believe they have sufficient evidence to pursue, but this, in practice, is not the only driving factor in the decision to prosecute.
The decision to prosecute also hinges on the particular office’s policies and objectives. The same chain of events could lead to the filing of criminal charges in one jurisdiction, but a decision not to prosecute in another. While office policies and objectives may lead to more decisions to charge than not to charge at the outset, an experienced criminal defense attorney knows that prosecutors will respond to potential weaknesses in cases at later stages in the case, possibly leading to dismissals or increased bargaining power.