"G-d said: What have you done? The voice of your brother's blood is screaming to Me from the ground" (Breisheet 4:10).
G-d approached Cain after the murder of Hevel with a question, hoping that Cain would do teshuva for the murder of his brother. Did he? The text is unclear: "And Cain said to G-d: gadol avoni mi'n'so, [Is?]my sin is too great to bear" (Breisheet 4:13). Whether this is a question or statement of fact is unclear; without hearing the tone of voice that Cain used, we are left in the dark.