The U.S. Secret Service is investigating how a gunman armed with an AR-style rifle was able to get close enough to shoot and injure former President Donald Trump at a rally Saturday in Pennsylvania, a monumental failure of one the agency’s core duties. The gunman, who was killed by Secret Service personnel, fired multiple shots at the stage from an “elevated position outside of the rally venue,” the agency said. An Associated Press analysis of more than a dozen videos and photos taken at the Trump rally, as well as satellite imagery of the site, shows the shooter, was able to get astonishingly…