Which screenplay by Kenneth Ross  was directed by Fred Zinnemann?
Black Sunday (1977 film):  Ross had previously written the screenplay for "The Day of the Jackal", a similar plot-driven political thriller. The Day of the Jackal (film): The Day of the Jackal is a 1973 Anglo-French political thriller film directed by Fred Zinnemann and starring Edward Fox and Michel Lonsdale.
The Day of the Jackal