In 1964, Gregory played a role in the search for three missing civil rights workers, James Chaney, Andrew Goodman, and Michael Schwerner, who vanished in Philadelphia, Mississippi. After Gregory and members of CORE met with Neshoba County Sheriff Lawrence A. Rainey, Gregory became convinced that the Sheriff's office was complicit. With cash provided by Hugh Hefner, Gregory announced a $25,000 reward for information. The FBI, which had been criticized for inaction, eventually followed suit with its own reward, and the rewards worked. The bodies of the three men were found by the FBI 44 days after they disappeared.