The largest all-masonry fort in the United States, Fort Jefferson (named after Thomas Jefferson) was constructed with more than 16 million bricks between 1846 and 1875 on Garden Key, which lies approximately 70 miles West of Key West. A federal outpost during the Civil War, Fort Jefferson held more than 500 prisoners by 1865 and in July of that year added Dr. Samuel Mudd, Edmund Spangler, Samuel Arnold and Michael O’Laughlen – all of whom had been convicted in the assassination of President Abraham Lincoln. Mudd was eventually pardoned by President Andrew Johnson after treating the victims of a yellow fever epidemic at Fort Jefferson in 1867. The Prisoner of Shark Island (1936), which starred Warner Baxter and Gloria Stuart, was loosely based on the life of Mudd. In 1935, President Franklin D. Roosevelt designated the area as Fort Jefferson National Monument and the fort was listed on the National Register of Historic Places in 1970. Fort Jefferson lies within the 64,701-acre Dry Tortugas National Park, which is accessible only by boat or seaplane.