KABUL, Afghanistan — A deadly traffic accident involving U.S. troops sparked a riot in the Afghan capital on Monday, with U.S. and Afghan security forces firing on protesters, police and witnesses said. At least five people were killed and 60 injured. Hundreds of protesters marched on the palace of U.S.-backed President Hamid Karzai in the city center after the incident, shouting "Death to Karzai! Death to America!" Gunfire was also heard near the U.S. Embassy. The staff was moved to a secure location within the heavily fortified compound, said Chris Harris, an embassy spokesman