Elephants can remember - Agatha Christie

Hercule Poirot is determined to solve an old husband and wife double murder that is still an open verdict...Hercule Poirot stood on the cliff-top. For here, many years earlier, there had been a tragic accident -- the broken body of a woman was discovered on the rocks at the foot of the cliff. This was followed by the grisly discovery of two more bodies -- a husband and wife -- shot dead. But who had killed whom? Was it a suicide pact? A crime of passion? Or cold-blooded murder? Poirot delves back into a crime committed 15 years earlier and discovers that, when there is a distinct lack of physical evidence, it's just as well that 'old sins leave long shadows'!