The carnage — the chaos, the cries and the death of a woman — began to unspool over a few stunning seconds on Saturday: James Alex Fields Jr., the police said, aimed his sleek Dodge Challenger and slammed it into a crowd in Charlottesville, Va.

Although the crash was a grisly coda to a day of clashes between white nationalists and their opponents, family members, acquaintances and internet posts suggested that Mr. Fields had mostly gone unnoticed by the authorities and researchers, even as he trafficked in radical views and unnerving behavior long before the outbreak of violence.

As a young man in Kentucky, he sometimes espoused Nazi ideology at school. A military career ended in less than four months. On Saturday, before the crash that left Heather D. Heyer, 32, dead, and 19 others hurt, he stood in Charlottesville, donned a white shirt and clutched a shield that bore a symbol of the so-called alt-right. He is expected to appear in court on Monday to face an array of charges, including a count of second-degree murder; the federal authorities have opened a civil-rights inquiry.

Mr. Fields spent most of his life in northern Kentucky and was raised near Cincinnati. His father was killed in a car accident in 1996, months before he was born.

“He was a very quiet little boy,” said an aunt, Pam Fields. “We’re just treating this as a family issue. We’re devastated as a family, and we really are praying for the victims and their families, and we are so sorry that this happened.”

Mr. Fields’s mother, Samantha Bloom, told The Toledo Blade that she did not regularly discuss politics with her son and that he had not expressed extremist views. But others who knew Mr. Fields, especially from his teenage years, said that his demeanor and opinions had troubled them for years.

“On many occasions there were times he would scream obscenities, whether it be about Hitler or racial slurs,” a woman who attended middle school with Mr. Fields in Florence, Ky., said in an email on Sunday.

The woman, who requested anonymity because she feared retaliation, said Mr. Fields “mostly kept to himself” and “didn’t start fights or try to fight.” But she described him as “exceptionally odd and an outcast to be sure.”

“He wasn’t afraid to make you feel unsafe,” said the woman, who was among the students who said Mr. Fields had made them feel unnerved.

As a freshman at Randall K. Cooper High School he wrote a report that, one teacher recalled, fell “very much along the party lines of the neo-Nazi movement.”

“A lot of boys get interested in the Germans and Nazis because they’re interested in World War II,” the teacher, Derek Weimer, told The Cincinnati Enquirer. “But James took it to another level.” Mr. Fields was “a very bright kid but very misguided and disillusioned,” Mr. Weimer said.

Mr. Fields enlisted in the Army after he graduated from high school in 2015, but military records show that his service lasted less than four months. Lt. Col. Jennifer Johnson, an Army spokeswoman, said Mr. Fields had been “released from active duty due to a failure to meet training standards.” Mr. Fields, Colonel Johnson said, “was never awarded a military occupational skill nor was he assigned to a unit outside of basic training.”

The sources of Mr. Fields’s ideology were unclear on Sunday, but his Facebook page included memes and symbols associated with the far right.

Mr. Fields was generally quiet, acquaintances said, and filled his time playing video games and working at a local grocery store. He and his mother eventually moved to the Toledo, Ohio, area, and Mr. Fields, a registered Republican, voted in the March 2016 presidential primary. By Saturday, he had driven his Challenger, his first car, to Charlottesville. He had told his mother that he was going to an event for the alt-right, a far-right fringe movement that embraces white nationalism.

“I don’t really understand what the rally was about or anything,” she told The Blade. “I didn’t know it was white supremacists,” she said. “I thought it had something to do with Trump. Trump’s not a supremacist.”

But in Charlottesville, Mr. Fields stood among men believed to be associated with Vanguard America, a group whose manifesto declares that “a government based in the natural law must not cater to the false notion of equality.” After the crash, it sought to distance itself from Mr. Fields.

“The driver of the vehicle that hit counter-protesters today was, in no way, a member of Vanguard America,” the group said in a statement on Twitter. “All our members had been safely evacuated by the time of the incident. The shields seen do not denote membership, nor does the white shirt. The shirts were freely handed out to anyone in attendance.”