On April 5, 1956, an assailant threw sulphuric acid into Riesel's face as he was leaving Lindy's (a famous restaurant in Manhattan). Riesel had been reporting on corruption in the International Union of Operating Engineers and its then-President, William C. DeKoning, Jr. He had recently alleged that DeKoning was conspiring with Joseph S. Fay (a convicted labor racketeer and extortionist) to re-establish his father, William C. DeKoning, Sr. (who had recently been freed from prison after serving a sentence for extortion) as president of the union. Although Riesel had received numerous death and other threats over the past few months, he had dismissed them as the work of "cranks."