Federal criminal cases may be brought by the U.S. Attorney's Office, the Criminal Division of the U.S. Department of Justice, or other authorized law enforcement agencies, such as the U.S. Marshals Service (USMS). The 93 U.S. attorneys serve as the chief federal law enforcement officers within their respective districts. Investigations are most commonly referred to a U.S. attorney by a federal investigative agency, including the Federal Bureau of Investigation (FBI), Drug Enforcement Administration (DEA), United States Citizenship and Immigration Services (USCIS), Customs and Border Protection (CBP), Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), and U.S. Secret Service. Investigations are also referred by a state or local investigative agency. Investigations may also be initiated—and cases brought directly—by U.S. attorneys or by the Criminal Division of the U.S. Department of Justice.