Al Capone Mabel Walker Willebrandt recognized that mob figures publicly led lavish lifestyles yet never filed tax returns, and thus could be convicted of tax evasion without requiring hard evidence to get testimony about their other crimes. She tested this approach by prosecuting a South Carolina bootlegger. In 1927, the Supreme Court ruled in "United States v. Sullivan" that illegally earned income was subject to income tax; Justice Oliver Wendell Holmes Jr. rejected the argument that the Fifth Amendment protected criminals from reporting illegal income. The IRS special investigation unit chose Frank J. Wilson to investigate Capone, with the focus on his
Al Capone suspected of being involved. Torrio headed an essentially Italian organized crime group that was the biggest in the city, with Capone as his right-hand man. He was wary of being drawn into gang wars and tried to negotiate agreements over territory between rival crime groups. The smaller North Side Gang led by Dean O'Banion (also known as Dion O'Banion) was of mixed ethnicity, and it came under pressure from the Genna brothers who were allied with Torrio. O'Banion found that Torrio was unhelpful with the encroachment of the Gennas into the North Side, despite his pretensions to be a settler