What was James Randi's earliest investigation?

James Randi's earliest investigation focused on the claims of a well-known psychic named Peter Popoff in the 1980s. Randi used hidden microphones and other techniques to expose Popoff's deceptive practices, revealing that he was receiving information about audience members through an earpiece from his wife. This investigation garnered significant media attention and marked the beginning of Randi's prominent role in debunking paranormal claims and promoting scientific skepticism.