(Reuters) - A handgun was found in the backpack of a 7-year-old boy at a New York City public elementary school on Thursday, triggering a brief lockdown amid heightened concern about gun violence in U.S. schools, officials said. New York City Police Department spokesman John Grimpel said authorities are investigating how the unloaded .22-caliber handgun ended up in the child's backpack. Police also found an ammunition clip and a flare gun in the bag belonging to the second-grade student, Grimpel said. ...