Atlanta police have video from cameras positioned at three locations near Saturday's robbery and shooting.
Lt. Keith Meadows said on Thursday that police believe "at least three, and possibly four" people were involved.
Meadows said the video produces a clear image of the suspected robber, but not the man who shot the 38-year-old Forrest multiple times in his back less than three minutes later. Meadows said the robber took Forrest's championship ring and Rolex watch.
Forrest's funeral is planned for Monday in Lithonia, near Atlanta.