The young man was pacing around the edges of the Donald Trump campaign rally, shouldering a big backpack and peering into the lens of a rangefinder toward the rooftops behind the stage where the former president would stand within the hour.

His behavior was so odd, so unlike that of the other rallygoers, that local law enforcement took notice, radioed their concerns and snapped a photo. But then he vanished.

The image was circulated by officers stationed outside the security perimeter on that hot, sunny Saturday afternoon. But the man didn’t appear again until witnesses saw him climbing up the side of a squat manufacturing building that was within 135 meters (157 yards) from the stage.

That’s where he opened fire, six minutes after Trump began speaking, in an attempt to assassinate the presumptive GOP presidential nominee. The gunman killed one rallygoer and seriously wounded two others. Trump suffered an ear injury but was not seriously hurt, appearing just days later at the Republican National Convention in Milwaukee with a bandage over the wound.