The so-called Mississippi Burning Murders were the murders of James Chaney, Andrew Goodman, and Michael Schwerner in June 1964. These murders were an attempt by the KKK to prevent African Americans in Mississippi from exercising their rights and registering to vote.
Wikipedia
Murders of Chaney, Goodman, and Schwerner
Video
The 1964 murders of Schwerner, Chaney, and Goodman (Jim Crow Museum)
Further Reading
Mississippi Burning (FBI)
Murder in Mississippi (PBS)
“Mississippi Burning” murders (CBS)
9 Things You Should Know About the ‘Mississippi Burning’ Murders (The Gospel Coalition)