While each state has different laws and the federal government has its own set of laws, some crimes fall under both a state and federal law. The federal government has priority to charge these overlapping crimes, and can even take the case from the state government after the person has already been charged.
In deciding whether to take such a case, the federal government focuses on factors such as the complexity of the case and the types of resources needed to prosecute it. In both systems, the government must prove the person’s guilt beyond a reasonable doubt.
It is a common misconception that the federal system has much harsher sentencing laws than the state system. The main reason that federal charges tend to produce harsher sentences is that the federal government tends to produce stronger cases. Federal prosecutors generally have much lower caseloads than state prosecutors, allowing them to devote more time to each case. The arrest process is usually different as well. In the state system, the local police department usually makes an arrest if there is only probable cause to do so, and leaves the bulk of the investigative work to the District Attorney’s Office. Often, a good deal of evidence isn’t gathered until after the arrest has already been made. Conversely, the federal government usually does months or years of investigation before even making an arrest, and obtains much of the evidence against the person before the arrest is made. This makes for a stronger case for the prosecution from the outset.