Mr. Green died March 28 at 91. (The Washington Post )

Bill Green, a journalist and university official who spent one momentous year as ombudsman of The Washington Post, where he conducted an investigation into a story by reporter Janet Cooke about an 8-year-old heroin addict that won a Pulitzer Prize before it was exposed as a fraud, died March 28 at his home in Durham, N.C. He was 91.

The cause was complications from surgery, said a daughter, Audrey Green.

Mr. Green was an editor of small-town newspapers in his native North Carolina and later worked for the U.S. Information Agency and as a public affairs officer for NASA during the Apollo space program.

He was a faculty member and top official for Duke University before coming to The Post as the newspaper’s ombudsman for a one-year term that began in September 1980.

“When I discussed taking this job with The Post, I asked what the point of an ombudsman was,” Mr. Green told the Duke University alumni magazine in 2003. “They said it was the reader’s representative. And they put no constraints on it.”

His tenure became notable when he wrote one of the most damning and thorough critiques of malfeasance in modern journalism, revealing the depth of deception by Cooke and the carelessness of her editors that led to one of the most humiliating episodes in The Post’s history.

“It was an intense, exhausting experience, and not all that pleasant because The Washington Post is one of our great newspapers that had made a terrible mistake,” Mr. Green once told the Eastern Wake News in North Carolina. “It was a dark day in journalism.”

On Sept. 28, 1980, weeks after Mr. Green’s arrival at the newspaper, a story appeared on The Post’s front page, “Jimmy’s World,” under Cooke’s byline. It was a detailed, shocking account of an unnamed 8-year-old boy in Southeast Washington who was a heroin addict. Cooke described the boy, his home and his surroundings in vivid detail.

There was some skepticism about the story from the beginning, but Cooke’s writing was so compelling that The Post nominated the article for a Pulitzer Prize. On April 13, 1981, the Pulitzer board announced that “Jimmy’s World” had won journalism’s most prestigious honor.

City officials sought to find Jimmy, to no avail, and murmurs of doubt began to swell into a clamor. Top Post editors questioned Cooke about her sources, learning that no supervisor had ever asked for or learned the boy’s true identity.

Cooke’s story, and her personal history, began to crumble. Discrepancies arose about her educational background. She claimed fluency in several foreign languages but could not reply when executive editor Benjamin C. Bradlee spoke to her in French.

“You’ve got 24 hours to prove the ‘Jimmy’ story is true,” Bradlee told her, according to an account written by Mr. Green and published in The Post.

When another Post reporter drove through Southeast Washington with Cooke, she appeared unfamiliar with the area’s gritty street life. Nevertheless, under questioning by other Post staffers, including then-Metro editor Bob Woodward and city editor Milton Coleman, Cooke maintained that her story was essentially true.

Later, after a long conversation with David Maraniss, then the deputy Metro editor, Cooke confessed that the article was an invention.

“There is no Jimmy and no family,” she said, according Mr. Green’s investigation. “It was a fabrication. I did so much work on it, but it’s a composite. I want to give the prize back.”

Cooke, once a rising star at the paper, admitted the fraud and resigned. The Post returned the Pulitzer.

With the paper’s credibility at stake, Bradlee asked Mr. Green to investigate how The Post failed its readers and itself. Within days, Mr. Green had spoken to everyone with a role in the “Jimmy” story and Cooke’s hiring — everyone but Cooke herself, who refused to be interviewed.

“I conducted 40-some interviews,” he said in 2012. “Then, there was a matter of writing what I had collected. We used typewriters in those days, and I remember typing for 28 uninterrupted hours.”

His investigation appeared on The Post’s front page on April 19, 1981, less than a week after the Pulitzer was announced.

“Working almost around the clock,” Bradlee wrote in his 1995 autobiography, “A Good Life,” “Bill Green accomplished an incredibly difficult task: a no-holds-barred, meticulously reported account of what went wrong — 18,000 words spread over the front page and four full pages inside.”

In his gripping account, Mr. Green provided an inside view of how journalism works — and how it sometimes fails. His conclusions were unsparing.

“There’s enough blame to go around,’’ he wrote. “Ben Bradlee, the executive editor, was wrong, and Howard Simons, the managing editor, was wrong. Beginning, of course, with Janet Cooke, everybody who touched this journalistic felony — or who should have touched it and didn’t — was wrong.”

Bradlee offered to resign, but publisher Donald E. Graham refused to let him go.

Plans were put in place at The Post and in other newsrooms across the United States to apply rigorous new standards to reporting the news. Over time, Mr. Green’s story was seen as a model of accountability of journalism itself.

“I think that the speed and thoroughness of that report,” Graham said Wednesday, “were crucial in making clear to readers that The Post was trying to tell them the whole story.”

William Lester Green was born Nov. 11, 1924, in Asheville, N.C., and grew up on a farm near Zebulon, N.C. He flew reconnaissance missions for the Army Air Forces in Italy during World War II.

After graduating in 1949 from the University of North Carolina at Chapel Hill, he began his journalism career in Durham and later became editor of the Morganton News Herald and the Shelby Daily Star.

With USIA, he was a press officer in Bangladesh and South Africa, before moving to NASA.

In 1970, he became director of university relations at Duke under university president Terry Sanford, a former North Carolina governor and future senator, who also sought the Democratic nomination for president. Mr. Green taught journalism and established a noted visiting journalist program at Duke before retiring from the university as a vice president in 1986. He later spent three years on Sanford’s campaign and Senate staff.

Survivors include his wife of 65 years, the former Isabel Yates, of Durham; five children, Lisa Kelley of Suwanee, Ga., Claudia Green of Durham, Erick Green of Washington, N.C., Bryan Green of Old Fort, N.C., and Audrey Green of Chapel Hill; two brothers; a sister; eight grandchildren; and seven great-grandchildren.

In a set of 15 recommendations for the newspaper at the close of his 1981 investigation of the Cooke affair, Mr. Green concluded, “If the reporter can’t support the integrity of his or her story by revealing the name to his or her editor, the story shouldn’t be published. And if that safeguard prevents some news stories from appearing, so be it.”