Answer:
Muckrackers can be considered as fathers of investigative journalism. Their work is closely associated with the Progressive Era, in the early-1900s.
They exposed social ills, dirty business, nepotism and corruption in business and politics. President Theodore Roosevelt described the work of the Muckraker as highly necessary, but urged them to stay true to it. The public was following the revelations with growing interest, and eventually a movement (muckraking movement) developed, which resulted in many judicial investigations of the affairs and some legislative reforms.
The best-known literary work of the Muckraker era is The Jungle by Upton Sinclair. The investigative journalism was previously represented by Benjamin Flower, Ida Tarbell and Lincoln Steffens and influenced Sinclair strongly. But the book itself and its legislative consequences also fertilized this kind of journalism. Other well-known muckrakers in the Progressive Era were Samuel Hopkins Adams and Nellie Bly.