He had simply babbled on uttering empty phrases, letting slip a few enigmatic words and again reverting to incoherence.
He wondered every time at his sombre and enigmatic impression and, mistrusting himself, put off finding the explanation of it.
This stupid repetition was too incongruous in its ineptitude with the serious, brooding and enigmatic glance he turned upon his visitor.
The conviction grew stronger in him that if that enigmatic man of yesterday, that phantom sprung out of the earth, had seen everything, they would not have let him stand and wait like that.
"Do you know, Sonia," he said suddenly with conviction, "let me tell you: if I’d simply killed because I was hungry," laying stress on every word and looking enigmatically but sincerely at her, "I should be happy now.
There are no more uses of "enigma" in the book.
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As Churchill said about Russia, it is a riddle, wrapped in a mystery, inside an enigma.