Dismissed for 175 years as a fake, a letter threatening the assassination of President Andrew Jackson has been found to be authentic. And, says the director of the Andrew Jackson Papers Project at the University of Tennessee, the writer was none other than Junius Brutus Booth, father of Lincoln assassin John Wilkes Booth. Dan Feller and his staff solved the mystery of the July 4, 1835, letter to Jackson. The story of their investigation will be featured this summer on PBS' "History Detectives." The letter, which addressed Old Hickory as "You damn'd old Scoundrel," demanded that Jackson pardon two prisoners...