The full form of FBI is Federal Bureau of Investigation. The FBI is the domestic intelligence and security service of the United States and its principal federal law enforcement agency. It is an intelligence-driven and threat-focused national security organization with both intelligence and law enforcement responsibilities. The FBI is a primary law enforcement agency for the U.S. government, charged with gathering facts and evidence to solve and prevent crimes. It is headquartered in Washington, D.C., and maintains 56 field offices in major cities throughout the United States, and more than 400 resident agencies in smaller cities and areas across the nation.