On Dec. 14, 2012, a gunman with a semiautomatic rifle killed 20 first-graders and six educators at Sandy Hook Elementary School in Newtown, Connecticut, then took his own life as police arrived; the 20-year-old fatally shot his mother at their home before the school attack.

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On Dec. 17, 1903, Wilbur and Orville Wright of Dayton, Ohio, conducted the first successful manned, powered airplane flights near Kitty Hawk, North Carolina, using their experimental craft, the Wright Flyer.

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On Dec. 17, 1903, Wilbur and Orville Wright of Dayton, Ohio, went on the first successful manned powered-airplane flights, near Kitty Hawk, N.…