Thomas Crooks was acting strangely. Sometimes he danced around his bedroom late into the night. Other times, he talked to himself with his hands waving around.

These unusual behaviors intensified last summer, after he graduated with high honors from a community college. He also visited a shooting range, grew out his thin brown hair and searched online for “major depressive disorder” and “depression crisis.” His father noticed the shift — mental health problems ran in the family.

On the afternoon of July 13, Mr. Crooks told his parents he was heading to the range and left home with a rifle. Hours later, he mounted a roof at a presidential campaign rally in western Pennsylvania and tried to assassinate Donald J. Trump.

That scene has been etched into American history. After a bullet grazed Mr. Trump’s ear, he lifted his blood-streaked face, pumped his fist and shouted the words: “Fight! Fight! Fight!” Mr. Trump has said that God saved him in order to save America, and the White House recently unveiled a statue in the Oval Office commemorating the moment.

The near miss revealed alarming security lapses that allowed an amateur marksman barely out of his teens to fire at a former president less than 500 feet away. And it galvanized support for Mr. Trump, inspiring voters who saw him as a righteous hero triumphing in the face of smear campaigns, relentless prosecutions and even an attempt on his life.