“It’s my world,” he said, in his final words to the jury before stepping down. He said that his training — that an agent should never shoot without a clear target and a certainty of what lies beyond the target — kept him from firing because he saw that a state trooper was potentially in the crossfire. Despite being involved in numerous SWAT team arrests and other actions, he said he had never shot at anyone, ever.

In reaching a verdict, the jury will have to fill in the holes of what happened, or did not, on Jan. 26, 2106, in the intense and frantic minutes as Mr. Finicum, a spokesman for the militia occupiers, roared his Dodge Ram pickup toward a police roadblock. Mr. Finicum swerved into a snowbank as he approached the blocked highway, nearly striking an F.B.I. agent. He then got out of the truck, hands in the air, but was shot and killed after the state trooper said he saw Mr. Finicum reach for a weapon.

Prosecutors, by contrast, said Mr. Astarita was new to the elite Hostage Rescue Team that had been assigned to the Malheur takeover. They said the evidence supported the notion that Mr. Astarita fired the two shots, then panicked — either because of his ego or his embarrassment — and denied using his weapon. He then had to lie further to cover his first lie, they said.

Defense lawyers, however, said that the evidence pointed to an Oregon trooper, Trooper No. 1, whose name has been withheld by the court because of threats he faced after the shooting. Mr. Angeli told the jury that Trooper No. 1 fired the two mysterious shots, and lied about it because of the pressure and the investigation.

What also hung over the proceedings is the reality that prosecutors have had very little success convicting anyone who played a role in the Malheur takeover, or more broadly in the militia groups that led it. The Bundy brothers, Ammon and Ryan, and five of their followers were acquitted in late 2016, in the same federal courthouse in downtown Portland where Mr. Astarita was on trial. In late 2017, another federal case against the Bundy group, stemming from an armed standoff with law enforcement agents in Nevada over cattle grazing, collapsed in a mistrial after the judge said prosecutors had failed to turn over evidence, as required, that could help the defense.

The task faced by the jurors as they began to deliberate on Thursday centered on what was deeply uncertain. The forensic evidence at the core of the prosecution’s case said the trajectory of the bullet that struck Mr. Finicum’s truck after it spun into the snow bank came from his approximate location. But Mr. Astarita told the jurors things were in chaotic motion, and he could not be exactly sure where he was standing.