The agency's hidden tape recorders turned up almost nothing about communism. But they did reveal embarrassing details about King's sex life -- details the FBI was able to use against him.
Watch how the FBI tracked King's every move (CNN Video)
The FBI's interest in King intensified after the March on Washington in August 1963, when King delivered his "I have a dream speech," which many historians consider the most important speech of the 20th century. After the speech, an FBI memo called King the "most dangerous and effective Negro leader in the country."
The bureau convened a meeting of department heads to "explore how best to carry on our investigation [of King] to produce the desired results without embarrassment to the Bureau," which included "a complete analysis of the avenues of approach aimed at neutralizing King as an effective Negro leader."