Watch Now: Two decades of deadly gun violence in U.S. schools

At least 19 children and two adults were shot dead on May 24 when an 18-year-old gunman opened fire at their Texas elementary school, the latest in the United States’ relentless cycle of school mass shootings. In the years since Sandy Hook, the gun control debate in Congress has waxed and waned. Efforts by lawmakers to change US gun policies in any significant way have consistently faced roadblocks from Republicans and the influence of outside groups such as the NRA.