Reginald Fairfield

US Army Major General Reginald 'Reggie' Fairfield (retired) was the unofficial leader of Delta Green during the "Cowboy" era and revealed the links between the Greys and Majestic-12 to the group.
He was assassinated in his cabin on the shore of Fairfield Pond, Connecticut, by an NRO Delta team led by Adolph Lepus in 1994, shortly after sending out his infamous Final Report. There is some evidence he deliberately sacrificed himself, invoking the unnatural with his death, for unclear purposes.
His assassination led to Delta Green's reorganisation as a cell-based conspiracy, linked with modern communications technology and directed by Cell A.

Further Reading: John Scott Tynes, 'The Corn King' and 'The Lucky Ones' (author's reflections)