The Bureau of Investigation began operations with nine special agents (who were former Secret Service employees), 13 Peonage investigators, and 12 Examiners. At first, the BOI was responsible for investigating about 25 different kinds of crime: antitrust, interstate train robberies, copyright violations, land fraud, neutrality violations, peonage, crimes on the high seas, forgery, and crimes on government reservations.
The first major expansion of the BOI's jurisdiction came in June 1910 when the Mann ("White Slave"). Act was passed -it provided a tool by which the federal government could investigate criminals who evaded state laws but had no other federal violations. Jurisdiction was again increased in 1917 and 1918 (after World War I broke out) with the passage of the Espionage Act, the Selective Service Act, and the Deportation Statute. Finally, in October 1919, Congress passes the Motor Vehicle Theft Act, another tool used to prosecute criminals who evaded the law by crossing state lines.
Because of its steadily increasing investigative responsibilities, the BOI grew substantially during this time frame - by the end of the decade, the BOI employed more than 200 Special Agents and over 250 support personnel.