The gunman in July’s assassination attempt on former President Donald Trump gained access to the roof he fired from by getting atop an air-conditioning unit, a source briefed on the investigation into the shooting told NBC News on Tuesday.

Trump was shot in the ear on July 13 during a rally in Butler, Penn., one rallygoer was killed and two others were wounded before a Secret Service counter-sniper shot and killed the 20-year-old gunman.