The Weapon, the Hour, the Motive (1972)

Sinopsis: Don Giorgio, a handsome young priest and teacher in a convent, is involved with two lovers: Orchidea and Giulia. When he repents and vows to return to the Church, he is savagely beaten to death. Not long after, Giulia has her throat slashed. Inspector Boito falls in love with and prepared to marry Orchidea, who is also the main suspect. The key to solving the case seems to lie with an inquisitive boy called Ferruccio, who is kept locked up and drugged. He witnessed one of the murders, but will anyone believe his story?