Then his father leveled a threat: If Jackson, 18, reported him to the police, he would have no choice but to do his “duty” for his country and “do what he had to do.”

In interviews with investigators, Jackson Reffitt said his father told him: “If you turn me in, you’re a traitor. And you know what happens to traitors. Traitors get shot.”

But he had already reported his father to the F.B.I. weeks before the riot.

“He would always tell me that he’s going to do something big,” the younger Mr. Reffitt said in a phone interview on Saturday. “I assumed he was going to do something big, and I didn’t know what.”