Complex sentence: Marks' letter shows that the spy ring extended into Drowned Meadow beyond Setauket and Oyster Bay and was authenticated by historians Georgette Grier-Key and Mark Rothenberg.
Split into simpler sentences: Marks' letter shows that the spy ring extended into Drowned Meadow beyond Setauket and Oyster Bay. The significance of the letter was authenticated by historians Georgette Grier-Key and Mark Rothenberg.