The trial comes at a difficult time for the F.B.I., which has been repeatedly maligned by President Trump for investigating whether any of his associates colluded with Russia to interfere in the 2016 election. A conviction would be a black eye for the bureau and in particular the hostage team, which has been involved in some of the country’s most sensitive counterterrorism operations.

The F.B.I. director, Christopher A. Wray, praised the hostage team during an appearance last month before Congress, noting it had recently carried out more than two dozen missions. Created in 1983, the rescue team also has worked closely overseas with American military commandos.

This month, the judge dismissed two of the charges against Mr. Astarita, handing his defense a partial victory. But the judge also refused to allow defense lawyers to question an Oregon State Police trooper at the scene about who fired the shots. He thought one of his fellow troopers fired into Mr. Finicum’s truck but did not witness it. The judge said the trooper could testify only about what he saw.

Little evidence ties Mr. Astarita to the shooting. Authorities never found the bullets or the casings, and neither the troopers nor the agents saw Mr. Astarita discharge his gun. Grainy surveillance footage taken from an F.B.I. plane shows agents milling around after the shooting. One can be seen bending over and possibly picking up something.

The case essentially boils down to a computer model that points to Mr. Astarita as the possible shooter based on the trajectory of the bullets. Mr. Astarita’s lawyers call the model flawed, saying it is impossible to determine his exact position during the shooting.

They also believe that a state trooper standing near Mr. Astarita was most likely responsible.

During the confrontation, a pair of troopers fired a total of six shots. One shot three times into Mr. Finicum’s truck as it barreled down the road before crashing into a snowbank. Both troopers then opened fire on Mr. Finicum, 54, as he exited the truck, hitting him three times.

The trooper who fired at the truck, a veteran of the Oregon State Police and now a captain, was involved in at least three other shootings and cleared in all of them. Defense lawyers are expected to question the captain, but he will not be identified by name during the trial because of threats made on his life, the judge said.