S. Supreme Court. A federal defendant begins his case in the District Court, but can appeal decisions. Because there is an essential difference between the violation of state and local laws and the violation of federal laws, it would seem as if federal law enforcement agents would play a different role from state or local agents. Pop media portrayals of the heartless federal agent who is willing to sacrifice individuals for the greater good accentuate this difference. However, the reality is that there is a tremendous amount of cooperation between state, local, and federal law enforcement agencies. The agencies share the common goals of preventing, detecting, and punishing criminal behavior. However, each agent is confined to the roles of their jurisdiction. For example, local and state law enforcement agencies have de facto jurisdiction over most crimes. It is only when a crime violates federal law or occurs on federal property that federal agencies obtain jurisdiction. Therefore, one of the main differences between the roles of the different law enforcement agents is that federal agents are more likely to be involved in investigating large-scale criminal operations that violate statutes like RICO, are suspected to threaten national security, or involve the movement of people over state lines.

There are no clear-cut guidelines determining when a crime will be investigated or prosecuted as a federal matter rather than a state matter. Most criminals who violate federal laws will violate state laws in the process. Unless a crime occurs solely on federal property, there is nothing in the law to compel a state to forego investigation or prosecution. In fact, it is not a double-jeopardy violation for a defendant to be tried in both the state and federal systems for the same criminal behavior....

If a defendant is suspected of involvement in a broader-ranging criminal activity, such as child internet pornography, local law enforcement agencies will often work in concert with federal agencies. These defendants are likely to be tried in federal court because such investigations will often result in multiple defendants in different jurisdictions, rendering individual state prosecution piecemeal. However, there are other concerns that determine where a defendant is tried. For example, mass and serial murders often confess when they are apprehended. These confessions may be obtained by plea agreements which take certain penalties off the table. Therefore, a jurisdiction that was not part of the plea agreement and that desires a greater penalty could try the case. For example, recently a criminal defendant who killed the older members of a family and abducted two young children for sexual purposes struck a plea deal with the state, which removed the possibility of the death penalty. However, because he transported the children across state lines, he brought himself into federal jurisdiction and the U.S. Attorney is considering a federal prosecution so that the defendant will be death eligible.
It is necessary to have criminal systems that overlap. First, many criminal enterprises cross state lines, which means that state law enforcement would not be adequately equipped to enforce the laws in question. However, local and state agencies have to have the power to investigate those crimes, because they are often the first agency to learn about possible legal violations. Sometimes it is only state and local investigations that reveal that a federal law has been violated. Furthermore, some state defendants evade punishment because of local corruption. One goal of federal law enforcement is to detect and prevent local corruption, and dual jurisdiction helps ensure that guilty defendants will not be able to evade detection and punishment.