Rashi on Genesis 4:9/רש"י על בראשית ד׳:ט׳

1איה הבל אחיך WHERE IS ABEL THY BROTHER — thus entering into a friendly conversation with him: perhaps he might repent and say, “I have killed him, and sinned against You” (Genesis Rabbah 19:11).

2לא ידעתי I KNOW NOT — He became a deceiver of the Most High (i. e. he persuaded himself that he could deceive Him) (Numbers Rabbah 20:6).

3השומר אחי אנכי AM I MY BROTHER’S KEEPER — This is a question. Similarly, wherever ‘ה has a חטף פתח it introduces a question.