What was the main interest of muckrakers

The term muckraker was used in the Progressive Era to characterize reform-minded American journalists who attacked established institutions and leaders as corrupt. They typically had large audiences in some popular magazines. In the US, the modern term is investigative journalism — it has different and more pejorative connotations in British English 

The muckrakers played a highly visible role during the Progressive Era period, 1890s–1920s