James Ridgeway, a muckraking journalist who trained his investigative powers on the automotive and energy industries, far-right hate groups and the U.S. prison system, among other quarries that he pursued over his six-decade career, died Feb. 13 at a hospital in D.C. He was 84.

The cause was complications from shingles, said his son, David Ridgeway.

Mr. Ridgeway fashioned himself after writers such as Upton Sinclair, Lincoln Steffens and I.F. Stone, his nose ever to the ground for whiffs of corruption or misconduct that had evaded public scrutiny. Working the phones and his sources, and hitting the road to follow a story wherever it led, he earned a reputation in Washington and beyond as the quintessential investigative journalist.

“I have never met a more honest, meticulous, humble, productive reporter with such a thirst for justice,” Ralph Nader, the consumer activist and former presidential candidate, said in an interview. Mr. Ridgeway was determined, Nader said, to “get the story out, get the story out,” because “people have to know, people have to know.”

Nader was at the center of one of Mr. Ridgeway’s first major scoops, published in the New Republic in 1964. (Mr. Ridgeway was a staff writer for the magazine at the time and wrote over the years for publications including the Village Voice, where he was Washington correspondent for three decades, as well as Mother Jones, the Nation and the Guardian.)

Nader, who had not yet acquired his national profile as an advocate for consumer protections, had gathered copious reports of defects in American-made cars that together, he argued, revealed the auto industry’s pattern of prioritizing profits over safety.

Nader was nearly penniless, he said, and hitchhiked to Washington to make in-person cold calls to journalists in the hope of attracting coverage for his findings. After being repeatedly dismissed, he presented himself at the New Republic office in Dupont Circle and asked at the reception desk if a reporter would be willing to meet with him. He was directed to Mr. Ridgeway, then in his late 20s.

“He had curiosity — that was the key thing,” Nader said.

On Sept. 19, 1964, the New Republic published an article by Mr. Ridgeway titled “Car Design and Public Safety.” It opened with a discussion of the Chevrolet Corvair, a General Motors model that was the subject of numerous lawsuits and consumer complaints about its safety. From there, with Nader as its principal source, the article broadened to highlight an overall lack of regulation in the auto industry.

“Each year the federal government spends millions in aircraft accident prevention research and up to $100,000 per victim to find the causes of airplane accidents,” Mr. Ridgeway wrote. “The design of every airplane down to every nut and bolt is checked for safety by the government. When it comes to automobiles, the government spends less than a nickel per casualty to find safer designs for automobiles, and there is no regulation whatever.”

Nader described the article as a turning point in his efforts to force change upon automakers. “People at The Washington Post and Washington Star would read the New Republic,” he said. “When they saw that article, they knew it was a news story.”

Mr. Ridgeway subsequently reported on efforts by automakers to harass and discredit Nader, who in 1965 published the seminal volume “Unsafe at Any Speed: The Designed-in Dangers of the American Automobile.” His work, along with Mr. Ridgeway’s, helped spur passage of the National Traffic and Motor Vehicle Safety Act of 1966, which marked a new era in oversight of safety on U.S. roadways.

Mr. Ridgeway was the author of several books on the coal, gas and oil industries and the environmental degradation that resulted from their operation. Among them were “The Politics of Ecology” (1970) and “The Last Play: The Struggle to Monopolize the World’s Energy Resources” (1973).

“Ridgeway is important because, apart from Nader and his team, he is apparently the only honest muckraker regularly raking environment muck,” Jerome Kretchmer, an environmental official in New York City, wrote in a Post review of “The Politics of Ecology.” He described Mr. Ridgeway as “one of our better journalists, radical in temperament and analysis, more traditionally reformist in his prescriptions.”

In his book “The Closed Corporation: American Universities in Crisis” (1968), Mr. Ridgeway documented the emergence of what he described as the “university industry,” in which institutions of higher learning morphed into sprawling conglomerates with tentacles reaching into business, real estate and government interests.

Later in his career, he delved into the Ku Klux Klan, the Aryan Nations and neo-Nazis for his 1990 book “Blood in the Face.” (The book became a documentary film released the following year, one of several films that Mr. Ridgeway helped produce.)

“Increasingly,” he wrote in the book, “race seems to lie just below the surface of nearly every political debate, and the opinions of the extreme right have been voiced by mainstream figures on both the political and cultural scenes. . . . The parameters of acceptable discourse and behavior have broadened, making room for more and more openly racialist viewpoints.”

An updated version of the book is slated for release in June.

James Fowler Ridgeway was born in Auburn, N.Y., on Nov. 1, 1936. His father was a historian at Wells College, in nearby Aurora, and worked for the State Department during World War II, bringing the family to Washington.

Mr. Ridgeway was a 1959 English graduate of Princeton University, where he was editor of the Daily Princetonian. In the early years of his career, he founded the newspaper Mayday — later called Hard Times — with Andrew D. Kopkind and wrote for Ramparts magazine.

Besides his son, of Fort Washington, Md., survivors include his wife of 54 years, Patricia Dodge Ridgeway of Washington.

Mr. Ridgeway devoted the final years of his career to documenting the brutality of solitary confinement in prisons. In 2010, with a collaborator, Jean Casella, he started the website Solitary Watch, which publishes, among other content, letters from prison inmates. He co-edited the volume “Hell Is a Very Small Place: Voices from Solitary Confinement” (2016).

“I wanted to use the prisoners themselves as reporters,” Mr. Ridgeway told the New Yorker in 2016. “Of course, that’s taboo in the mainstream press, since we all know they’re liars and double dealers and escape artists.”

Mr. Ridgeway received many letters from inmates at a P.O. box he maintained near his home, making weekly trips to the post office with the aid of a walker to collect them.

“I am a journalist. I’ve been taught to report what I see and hear and know, and nothing else. These letters should be nothing more to me than documentary material,” Mr. Ridgeway wrote in a reflection on his work. But he was so moved by the accounts that he could not help but respond to as many correspondents as time allowed.

“They write back,” he recalled, “with a level of gratitude totally disproportionate to my lame missives.”