SEATTLE — When a U.S. Marshals task force killed a self-described antifa activist in Washington State in September, the Trump administration applauded the removal of a “violent agitator” who was suspected of murder. Last week, local investigators concluded a monthslong homicide inquiry with the announcement that the activist, Michael Reinoehl, had most likely fired at authorities first, effectively justifying the shooting.

But a review of investigation documents obtained by The New York Times suggests that investigators for the Thurston County Sheriff’s Office discounted key pieces of contradicting evidence that indicate Mr. Reinoehl may never have fired or pointed a gun.

While investigators found a spent bullet casing in the back seat of Mr. Reinoehl’s car, and pointed to that as evidence he probably fired his weapon, the handgun they recovered from Mr. Reinoehl had a full magazine, according to multiple photos compiled by Thurston County authorities showing Mr. Reinoehl’s handgun. The gun was found in his pocket.

The federally organized task force, made up primarily of local law enforcement officers from Washington, had been seeking to arrest Mr. Reinoehl for the Aug. 29 shooting death of a supporter of the far-right group Patriot Prayer during the summer’s raucous street protests over race and policing. The arrest operation quickly erupted into gunfire, and Mr. Reinoehl died in the street near his car in a residential neighborhood in Lacey, Wash.

The sheriff’s office in Thurston County, where the shooting occurred, was not part of the task force.

In announcing its conclusions, the sheriff’s office wrote that “witness statements indicate there was an exchange of gunfire, which was initiated by Reinoehl from inside his vehicle.” A spokesman, Lt. Cameron Simper, said that while investigators could not conclude for certain that Mr. Reinoehl had fired his weapon, he said it was “highly likely.”

But one of the witnesses that Thurston County investigators relied on to reach their conclusion that Mr. Reinoehl had fired his gun was an 8-year-old boy. His father, Garrett Louis, who had rushed to his son’s side at the time of the shooting, has consistently said he believed that officers opened fire first without shouting any warnings.

Of the two other witnesses who investigators cited to support the conclusion that Mr. Reinoehl fired his gun, one did not see it happen and the other was not sure.

Fred Langer, a lawyer representing Mr. Reinoehl’s family, said the law enforcement conclusions defy common sense.

“They are covering for themselves,” Mr. Langer said. “The physical evidence doesn’t support what they are saying.”

Mr. Reinoehl had been a consistent fixture at racial justice protests in Portland, Ore., last summer, carrying a gun as a volunteer security officer among the protesters and writing online that the protests were part of a war with the potential to “fix everything.” On Aug. 29, when a caravan of Trump supporters drove into downtown Portland, clashing with left-wing activists, Mr. Reinoehl was on the streets.

Video footage shot by bystanders appears to show that Mr. Reinoehl approached Aaron J. Danielson, the Patriot Prayer supporter, as Mr. Danielson walked through the area with a can of bear repellent and an expandable baton. Mr. Reinoehl appears to have shot Mr. Danielson, killing him, before running into the night. He later claimed in an interview with Vice News that he had fired in self-defense.

Five days after the shooting, Portland police issued a warrant for Mr. Reinoehl’s arrest on suspicion of murder. The Pacific Northwest Violent Offender Task Force, whose local law enforcement officers were deputized as federal marshals, traced Mr. Reinoehl’s path up to Washington State and prepared a plan to take him into custody.

The investigation by Thurston County investigators that was obtained by The Times provided key new details, including witness statements, from their monthslong inquiry into the events preceding Mr. Reinoehl’s death.

Officers believed that Mr. Reinoehl had a .380-caliber handgun, an AR-style rifle and a shotgun, according to the accounts they gave to investigators. They said they had received information — apparently from an informant — that Mr. Reinoehl had said he would not be taken alive. Officers described their concern that Mr. Reinoehl was associated with “antifa,” the loose network of activists who have mobilized to confront far-right groups and protest law enforcement violence.

On Sept. 3, the officers took up surveillance positions near the apartment where Mr. Reinoehl was staying, according to their statements. Once on the scene, their chosen radio frequency only worked for a few officers, leaving the others unable to communicate.

Just before 7 p.m., the team watched as Mr. Reinoehl exited the apartment and headed toward his vehicle. Sgt. Erik Clarkson of the Pierce County Sheriff’s Department, a senior officer on the scene, told the others “to let him drive if no one was close enough to interdict him,” but his command was not heard as a result of the radio problem, according to his statement.

Officer Michael Merrill of the Lakewood Police Department decided to move in, and gunned his Ford Escape toward Mr. Reinoehl’s parked Volkswagen Jetta.

No video has emerged to show what transpired next, and a murky mix of sometimes contradictory information has been used to explain it. None of the officers wore a body camera, nor were cameras mounted on their vehicles. One of the officers on the scene, a deputy U.S. marshal named Ryan Kimmel who did not fire his weapon, declined to provide a statement during the investigation.

James Oleole, a Pierce County sheriff’s deputy in the passenger seat of Officer Merrill’s Ford Escape, said that as law enforcement vehicles pulled up and officers announced themselves, Mr. Reinoehl was in the driver’s seat of his Jetta and made moves with his arms “consistent with the moves that someone makes when they are attempting to grab a gun they have on their person.”

Although he did not see a gun, Deputy Oleole said, he began firing his AR-15 rifle through his own windshield at Mr. Reinoehl. Officer Merrill, thinking the glass shards from the windshield meant he was under fire, exited the Ford Escape, saw what he believed was Mr. Reinoehl reaching for a gun, and also opened fire. A third officer, also from the Pierce County Sheriff’s Department, had followed the others in an S.U.V. and blocked Mr. Reinoehl’s Jetta from an angle. Also believing that Mr. Reinoehl was reaching for a gun, he opened fire with his 9-millimeter handgun.

As the officers unleashed a hail of bullets, a total of 40 in all, Mr. Reinoehl exited the Jetta, shielding himself, and ran for cover behind a truck parked behind him. The three officers reported that he was continuously reaching around his waistband or pocket. A Washington State Department of Corrections officer, who had arrived in a third vehicle, saw Mr. Reinoehl round the rear of the truck and begin to pull “a small dark item” from his pocket. That officer also fired, and Mr. Reinoehl fell.

Although no officer said Mr. Reinoehl shot at them, and only one described him raising something that might have been a gun, investigators concluded that Mr. Reinoehl had most likely fired a shot — pointing to a spent shell casing they found in the back seat of the Jetta that matched the .380-caliber handgun found in his pocket.

Investigators never found a bullet matching it amid the dozens sprayed around the scene, and all of the gunshots that pierced the Jetta’s front windshield were determined to be incoming rounds fired by officers. Lieutenant Simper of the Thurston County Sheriff’s Office said it was possible that Mr. Reinoehl fired through an open passenger-side window.

The final report also does not address the fact that the handgun’s six-round magazine was still full when officers recovered it. Lieutenant Simper said it was possible that Mr. Reinoehl had loaded an extra round in the chamber before firing and that the gun had malfunctioned and failed to load a round from the magazine after he took a shot.

To reach their conclusion that Mr. Reinoehl fired his gun, investigators also cited the accounts of three witnesses. One of them, Chad Smith, had initially told journalists that he saw Mr. Reinoehl shooting at officers but later said he did not see Mr. Reinoehl shooting. He reported to investigators that he believed that Mr. Reinoehl shot first because the first shot he heard sounded less powerful than later ones.

Another witness told investigators he believed there was an exchange of gunfire. The man, who asked not to be identified publicly, said in an interview on Friday that he could not be sure Mr. Reinoehl had fired a weapon.

Mr. Louis’s 8-year-old son told officers that Mr. Reinoehl was shooting at the agents. But when asked what kind of gun Mr. Reinoehl fired, he described it as “big” and “two-handed,” a description that did not match Mr. Reinoehl’s pocket-size handgun.

Mr. Louis said his children were taught that police officers were “heroes” but that the investigator who interviewed his son had phrased his questions in a way that prompted the boy to say that Mr. Reinoehl had fired his weapon.

“He initially told me for the first 24 hours that he didn’t know that guy had a weapon,” he said.