Countries around the world are organized in a variety of ways and consequently their legal systems differ as well. In a federal republic, there is a federal government, as well as a federation of states, each of which has its own government as well. As a result, in a federal republic, a person can be charged with a state crime or with a federal crime. Aside from the United States, other examples of federal republics include the United Mexican States, the Federal Republic of Brazil, the Federal Republic of Germany, and the Republic of India, among others. When a person violates a federal law, as opposed to a state law, it is considered a federal crime.
Within the United States, as well as other federal republics, the Constitution provides the framework for all laws. Both federal and state laws are subject to constitutional review. Although the states may enact laws specific to the states, they may not abridge or violate the protections afforded by the Constitution. A state law may afford more protection or additional rights than the constitution provides, but not less.
A federal crime in the United States is generally a crime that either happens on federal property, involves a high ranking government official or dignitary, or has a substantial relationship to interstate commerce. The "relationship to interstate commerce" element is the basis for most federal criminal statutes within the United States. Bank robbery, kidnappings that cross the state line, gun offenses, and drug crimes that are charged at the federal level are all based on the commerce element.
The law enforcement agency within the United States that is predominately responsible for investigating a federal crime is the Federal Bureau of Investigation (FBI). Other federal law enforcement agencies that investigate certain federal crimes include the Drug Enforcement Agency (DEA), the Bureau of Alcohol, Tobacco, and Firearms (ATF), and Immigrations and Customs Enforcement (ICE). The United States Attorney's Office, which is part of the United States Department of Justice, is responsible for prosecuting federal crimes.
While some crimes, such as terrorism, are solely within the federal government's jurisdiction, many other crimes may be charged as a state crime, a federal crime, or both. Many drug crimes, for example, could be prosecuted by the federal government if they chose to do so. As a practical matter, the federal government generally only chooses to prosecute large-scale drug operations when they have been a part of the investigation. Although the concept of double jeopardy is part of the American legal system, which forbids prosecution twice for the same crime, a person can be charged in both state and federal court for the same crime.