Signs of trouble were evident in the minutes before shots rang out at Donald Trump’s rally in Pennsylvania: Police had a report of a suspicious man pacing near the magnetometers and were apparently exchanging photos of the suspect. Witnesses pointed and shouted at an armed man on a nearby roof.

When a police officer climbed up to the roof to investigate, the gunman turned and pointed his rifle at him. But the officer did not — or could not — fire a single shot.