I'm going to quote from the wiki article as I had trouble with following the link. I was able to access it from google.
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A muckraker is, primarily, a reporter or writer who investigates and publishes truthful reports involving a host of social issues, broadly including crime and corruption and often involving elected officials, political leaders and influential members of business and industry. The term is closely associated with a number of important writers who emerged in the 1890s through the 1930s, a period roughly concurrent with the Progressive Era in the United States.