The Federal Bureau of Investigation was founded in 1908 when the Attorney General appointed an anonymous force of 34 Special Agents to be investigators for the Department of Justice. Before that, the DOJ had to borrow Agents from the U.S. Secret Service. In 1909, the Special Agent Force was renamed the Bureau of Investigation, and after countless name changes, it became The Federal Bureau of Investigation in 1935.
When the FBI was established, there weren't an abundance of federal crimes, so it investigated criminal acts that dealt with national banking, bankruptcy, naturalization, antitrust, peonage, and land fraud. In June of 1910, the FBI grew larger because the "Mann Act" (Made it a crime to transport women to other states for immoral reasons). The FBI could now prosecute people whom tried to flee over states lines. Because of its continued worth and effectiveness, the FBI's number grew to over 300 special agents and 300 support employees over the next few years. When the U.S. entered World War I, the FBI was given responsibility of Selective Service, Espionage, Draft violations, and Sabotage Acts. . The passage of the National Motor Vehicle Theft Act in 1919 made it even easier for the FBI to prosecute criminals.
In 1920, the gangster era began. This brought a new type of crime into play that had not been seen before. Criminals were kidnapping and robbing banks, both of which were not federal crimes at that time. In 1932 the passage of a federal kidnapping statute made it a criminal act. In 1934, many other federal criminal statutes were passed, and Congress gave Special Agents the authority to make arrests and to carry firearms. The FBI's size and jurisdiction during the second World War greatly increased and included intelligence matters abroad. At the end of World War II, and the introduction of the Atomic Bomb, the FBI began working on background security investigations for the White House and other government agencies, as well as
Cited: www.FBI.gov - The Federal Governments site on the FBI The FBI in Action - Greg Bosenowshi, C. 1998 The FBI and How It Works - Robert Stellinkski, C. 2001