SNCC and CORE helped organize the 1964 voter registration drive in Mississippi. This campaign, known as Freedom Summer or the Mississippi Summer Project, aimed to increase the number of registered Black voters in the state. The groups spent a week training mostly White student volunteers about the ugliness and dangers they would face that summer, including the high probability that the volunteers would face beatings and arrests. The SNCC and CORE leaders also emphasized that they would not change Mississippi in one summer, but that the campaign was the start of a long, protracted battle to end segregation in the state. Lewis, like many others, felt fear. He did not know how the Black and White workers would get along or how locals, especially those living in rural areas, would treat them.

Two White students from New York, Michael Schwerner and Andrew Goodman, and a local Black Mississippian, James Chaney, were among the first wave of Mississippi Summer volunteers. The three men disappeared on June 21, 1964, after investigating the burning of a church in Philadelphia, Mississippi. The authorities in Philadelphia refused to cooperate with Lewis and other SNCC members. An attorney claimed that the boys went on a short vacation and would be coming back soon.