Final answer:
The thief was Minister D―, and the captain found him by observing his actions and deducing his guilt.
Step-by-step explanation:
The thief was Minister D―. The captain found him by observing the Minister's actions and deducing that he had stolen the letter. The Minister pretended to read a similar letter and then swapped it with the stolen one while conversing with the person to whom the letter belonged. The captain saw the exchange but couldn't say anything at the time. The complexity of the situations and the methods used to identify or catch the thief are examples of dramatic tension and character interaction, often used in literature to engage the reader and develop the plot.