Death by Drowning

When Sir Henry Clithering, the retired commissioner, is visiting his friend near the village of St Mary Mead, everyone is saddened by the news of one young girl's death. Rose Emmott was found in the forest in the river near the mill. There were rumors that the girl was pregnant by an architect from London. She was afraid to tell her father about it. So the girl drowned herself. But the investigation shows that it was a murder. Rose's beloved becomes the prime suspect. Miss Marple knows the name of the real killer. But she has no proof. She writes the name of the culprit on a piece of paper and asks the commissioner for help. There is a witness who saw two men in the woods at the time of the murder. Only one of them has an alibi. Who is the criminal? Whose name is written on this piece of paper?