Fort Jefferson is a massive but unfinished coastal fortress. It is the largest masonry structure in the Americas, and is composed of over 16 million bricks. The Dry Tortugas are part of Monroe County, Florida, United States. The fort is located on Garden Key in the lower Florida Keys within the Dry Tortugas National Park, about 70 miles (110 km) west of the island of Key West. In July 1865 four special civilian prisoners arrived. These were Dr. Samuel Mudd, Edmund Spangler, Samuel Arnold, and Michael O'Laughlen, who had been convicted of conspiracy in the assassination of President Abraham Lincoln.

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