The Federal Bureau of Investigation (FBI) is an investigative arm of the United States Department of Justice (DOJ).
It is an intelligence agency which deals with specific crimes under federal statutes of USA. According to American federal law, FBI has investigative jurisdiction over more than 200 categories of federal crimes, and the broadest investigative authority of any law enforcement agency in America.
The mission of the bureau is multifaceted. It enforces US law by investigating violations of federal criminal law, protects US from foreign intelligence agencies, prevents terrorism in America, provides assistance to federal, state and international agencies and performs these duties according to public needs and is faithful to united states constitution.
The FBI gathers intelligence, which is presented to a US attorney or department of justice official who decides if prosecution or any other action is warranted.