John Douglas was an agent of Moriarty who in turn employed Daniel Gottlieb and other assassins for Moriarty. In "A Landmark Story", Sherlock texts Moriarty with Daniel Gottlieb's phone arranging a meeting at diner. Waiting outside, Sherlock sees Isaac Proctor waiting for the meeting. When Issac leaves Sherlock tails him, but is cut off by a train. Sherlock takes a series of photos, then goes home and edits them together, creating a picture of Douglas.