Early in 1887 Dew was transferred to Commercial Street police station in H Division (Whitechapel), where he was a detective constable in the Criminal Investigation Department during the Jack the Ripper murders.

In his memoirs, published fifty years later in 1938, Dew made a number of claims about being personally involved in the Ripper investigation. None of these claims have been confirmed by surviving police records, and some of them contradict known evidence in the case.

Dew claimed to know Mary Jane Kelly by sight. “Often I saw her parading along Commercial Street, between Flower and Dean Street and Aldgate, or along Whitechapel Road”, he wrote. “She was usually in the company of two or three of her kind, fairly neatly dressed and invariably wearing a clean white apron, but no hat.”

Dew also claimed to have been one of the first police officers on the murder scene, though none of the records mentioning those people who were present list his involvement. Dew wrote that he saw Kelly’s mutilated body in her room in Miller’s Court and that he regarded it as “the most gruesome memory of the whole of my Police career.”