Officials at the Federal Bureau of Investigation feared the novelist, whose work "definitely slanted" against capitalism, may have posed a threat to America during the 1950s and 1960s since he allegedly often spoke out against former U.S. Senator Joseph McCarthy, who spearheaded a movement to track down Soviet spies and sympathisers living in the country.
It was later determined Bradbury did not have "informant potential", so he was never properly questioned.
The revelation was discovered in recently-declassified documents obtained by the Associated Press.
The Fahrenheit 451 writer passed away in June (12).