The criminal justice process officially starts when a police officer places a person under arrest, which means they are taken into police custody. There are usually three instances in which a police officer can make an arrest.
First, if a police officer observes a crime, the individual can be arrested. For example, an officer may see someone shoplifting. The officer can arrest that person immediately. A police officer may see a driver he suspects is under the influence of alcohol, and if the individual is found to be intoxicated, the police officer can arrest that driver.
The second situation in which a police officer can make an arrest is if he or she has “probable cause”. That means the officer has a reasonable belief that a person has committed or is about to commit a crime, based on circumstances and facts. For example, an officer may see a person leaving a bank wearing a mask and carrying a gun and a sack full of cash. The police officer can arrest the person based on probable cause that he or she robbed the bank.
The third situation in which police can make an arrest is if an arrest warrant has been issued. An arrest warrant is a legal document signed by a judge, authorizing an arrest to be made.