他突然辞职的真正动机仍是个谜。

The true motive behind his sudden resignation remains a ___puzzle.

考古学家仍在拼凑这个古老文明崩溃的谜团。

Archaeologists are still piecing together the ___puzzle of the ancient civilization's collapse.

相互矛盾的证据给调查人员出了一个真正的难题。

The inconsistent evidence presented a real ___puzzle for the investigators.