muckraker

noun3

1. •
   One who investigates and exposes issues of corruption that often violate widely held values; e.g. one who exposes political corruption or the poor conditions in prisons.
2. •
   (US, historical) One of a group of American investigative reporters, novelists and critics of the Progressive Era (the 1890s to the 1920s).

   • "Lady," says he, "the goil's nutty! You got a bughouse patient on your bands! This here talk about the white-slave traffic, ma'am… it's all the work o' these magazine muckrakers!" — 1912
3. •
   (UK, disapproving) A sensationalist, scandalmongering journalist, one who is not driven by any social principles.