A young man calling himself Mohamed al-Alawi appeared in a YouTube video in August. He described himself as an investigative journalist in Egypt with a big scoop: The mother-in-law of Ukraine’s president had purchased a villa near Angelina Jolie’s in El Gouna, a resort town on the Red Sea.

The story, it turned out, was not true. Ukraine denied it, and the owner of the villa refuted it. Also disconnected from reality: Alawi’s claim to being a journalist.

Still, his story caromed through social media and news outlets from Egypt to Nigeria and ultimately to Russia — which, according to researchers, is where the story all began.

The story seemed to fade, but not for long. Four months later, two new videos appeared on YouTube. They said Mohamed al-Alawi had been beaten to death in Hurghada, a town about 20 miles south of El Gouna. The suspected killers, according to the videos: Ukraine’s secret service agents.

These claims were no more factual than the first, but they gave new life to the old lie. Another round of posts and news reports ultimately reached millions of internet users around the world, elevating the narrative so much that it was even echoed by members of the U.S. Congress while debating continued military assistance to Ukraine.

Ever since its forces invaded two years ago, Russia has unleashed a torrent of disinformation to try to discredit Ukraine’s leader, Volodymyr Zelensky, and undermine the country’s support in the West.

This saga, though, introduced a new gambit: a protracted and elaborately constructed narrative built online around a fictitious character and embellished with seemingly realistic detail and a plot twist worthy of Netflix.

“They never brought back a character before,” said Darren Linvill, a professor and director of the Media Forensics Hub at Clemson University, who has extensively studied Russian disinformation.

The campaign shows how deftly Russia’s information warriors have shifted to new tactics and targets as the war in Ukraine has dragged on, just as Russian forces on the ground in Ukraine have adjusted tactics after devastating battlefield losses.

Groups with ties to the Kremlin continue to float new narratives when old ones fail to stick or grow stale, using fake or altered videos or recordings and finding or creating new outlets to spread disinformation, including ones purporting to be American news sites.

A video appeared on TikTok last month claiming to show a Ukrainian doctor working for Pfizer accusing the company of conducting unlawful tests on children. On the social network X, a man claiming to be an associate producer for Paramount Pictures spun a tale about a Hollywood biopic on Mr. Zelensky’s life.

The tale attributed to Mohamed al-Alawi is not even the only baseless allegation that Mr. Zelensky had secretly purchased properties abroad using Western financial assistance. Other versions — each seemingly tailored for a specific geographic audience — have detailed a mansion in Vero Beach, Fla., and a retreat in Germany once used by Joseph Goebbels, the Nazi minister of propaganda.

The Russians have “demonstrated adaptability through the war on Ukraine,” Microsoft wrote in a recent report that disclosed Russia’s fraudulent use of recorded messages by famous actors and celebrities on the Cameo app to try to smear Mr. Zelensky as a drug addict.

Even when debunked, fabrications like these have proved exceedingly difficult to extinguish entirely.

YouTube took down the initial video of the character Mohamed al-Alawi, linking it to two other accounts that had previously violated the company’s policies. The accusation still circulates, however, especially on platforms, like X and Telegram, that experts say do little to block accounts generating inauthentic or automated activity. Some of the posts about the video appear to have used text or audio created with artificial intelligence tools; many are amplified by networks of bots intended to create the impression that the content is popular.

What links the narratives to Russia is not only the content disparaging Ukraine but also the networks that circulate them. They include news outlets and social media accounts that private and government researchers have linked to previous Kremlin campaigns.

“They’re trolling for a susceptible (and seemingly abundant) slice of citizens who amplify their garbage enough to muddy the waters of our discourse, and from there our policies,” said Rita Katz, the director of the SITE Intelligence Group, an American company that tracks extremist activity online and investigated the false claims about the villa.

The video first appeared on Aug. 20 on a newly created YouTube account that had no previous activity and almost no followers, according to the Institute of Strategic Dialogue, a global nonprofit research organization in London, which traced the video’s spread.

The man appeared in a poorly lit room reading from his computer screen, which was reflected in his thick glasses. He appeared to be a real person, but it has not been possible to verify his actual identity. No one by the name of Mohamed al-Alawi appears to have produced any previous articles or videos, as would be expected of a journalist. According to Active Fence, an internet security company, the character has no educational or work history, and no network of friends or social connections online.

The video, though, showed what purported to be photographs of a purchase contract and of the villa itself, creating a veneer of authenticity for credulous viewers. The property is, in fact, part of a resort owned by Orascom Development, whose website highlights El Gouna’s “year-round sunshine, shimmering lagoons, sandy beaches and azure waters.”

An article about the video’s claim appeared two days later as a paid advertisement, or branded content, on Punch, a news outlet in Nigeria, as well as three other Nigerian websites that aggregate news and entertainment content.

The article had the byline of Arthur Nkono, who according to internet searches does not appear to have written any other articles. The article quoted a political scientist, Abdrulrahman Alabassy, who likewise appears not to exist except in accounts linking the villa to the corrupt use of Western financial aid to Ukraine. (Punch, which later removed the post, did not respond to requests for comment.)

A day later, the claim made its first appearance on X in a post by Sonja van den Ende, an activist in the Netherlands, whose articles have previously appeared on propaganda outlets linked to the Russian government, according to the Institute for Strategic Dialogue. (She also served as an election observer in an occupied territory of Ukraine during Russian parliamentary elections in September.)

Within days, reports about the villa appeared on X in French and Romanian, and in English on three different Reddit forums.

According to Roberta Duffield, director of intelligence for Blackbird.AI, an internet security company, nearly 29 percent of the accounts amplifying the reports appeared to be inauthentic bots, an unusually high number that would normally indicate a coordinated campaign.

Eight days after the video appeared, Russia state television networks like Channel One, Rossiya 24 and RT (in Arabic and German) reported it as a major revelation uncovered by a renowned Egyptian investigative journalist.

The story seemed to stall there. Naguib Sawiris, the scion of the Egyptian family that owned the development, curtly denied the sale in a reply on X.

And no more was heard from or about the character called Mohamed al-Alawi — until late December.

That was when two new videos emerged on a YouTube channel called “Egypt News,” claiming that he was dead.

The channel had been created the day before. One video showed a man identified as Alawi’s brother, Ahmed, answering questions from another man.

The police, he said, told him that they suspected his brother had been beaten to death by “Ukrainian special forces who acted on behalf of President Zelensky or another high-ranking official.”

He spoke with his hand cupped over his face to obscure his identity. The other video showed what was said to be the site of an attack, though the images were indistinct. “I can’t tell you anything else,” he said in the video, which YouTube later removed. “I’m afraid for my family.”

The video also tried to explain away some of the obvious holes in the initial story, including why there was no evidence online of Alawi’s previous work. “It was his first big assignment,” the man said.

The new episode spread as the first video had. A day later, an article about the death appeared on an obscure website created last year called El Mostaqbal, a name similar to but unrelated to the actual news organization in Lebanon.

“A reporter who announced that Zelensky’s mother-in-law brought a luxury villa has died under mysterious circumstances,” the headline read. Other reports that followed dropped any uncertainty and began referring to his “murder.”

In fact, Egypt’s Ministry of the Interior said there were no reports or evidence that anyone resembling the man in the video had been “subjected to harm.” The statement went on to note that the property itself had not been sold.

Still, according to the Institute for Strategic Dialogue, posts about the supposed killing were viewed a million times on X on Dec. 25.

It also appeared on the website of the Middle East Monitor, or MEMO, operated by a well-known nonprofit organization in London and financed by the government of Qatar. A journalist who once reported from Moscow for The Telegraph of London, Ben Aris, cited it at length on the platform, though, when challenged, he said he had just made note of the rumor. “I don’t have time to check all this stuff myself,” he wrote.

It appeared in English on a site, Clear Story News, that Mr. Linvill of Clemson’s Digital Media Hub had previously linked to Russia’s disinformation efforts. (The site lists no contact information)

Mr. Linvill described the process as a form of “narrative laundering” — moving false claims from unknown or not credible sources to ones that, to the unwitting at least, seem more legitimate.

More Elaborate Narratives

The Institute for Strategic Dialogue studied three other complex narratives about Ukraine, as well.

One featured a French journalist who claimed that the son of George Soros — a regular target of Russian and far-right political attacks — had secretly acquired land for a toxic waste dump in Ukraine. An unnamed doctor in Africa said in another that an American medical charity, the Global Surgical and Medical Support Group, was harvesting the organs of wounded Ukrainian soldiers for transplants for NATO officers.

Then there was the case of a man calling himself Shahzad Nasir, whose profile on X identifies him as a journalist with Emirates 24/7, an English-language news outlet in Dubai, though he has no apparent bylines on the site.

In November, he claimed that cronies of Mr. Zelensky bought two yachts — Lucky Me and My Legacy — for $75 million. His evidence, like Mohamed al-Alawi’s, includes photographs of the vessels and purported purchase agreements.

In fact, as the BBC documented in December, the yachts had not been purchased and remained for sale. Despite numerous efforts by fact checkers to dispel it as rumor, the claim circulated extensively.

Last month, the character Nasir reappeared in another video. This time he had a new version of the tale, claiming that the purchases had been scuttled after he exposed the secret deal.

The ramifications of these campaigns are difficult to measure precisely. There are signs, though, that they resonate even when proved false.

Senator J.D. Vance, a Republican of Ohio and an outspoken critic of Ukraine aid, seemed to embrace the claim in December during an interview on “War Room,” the podcast hosted by Stephen K. Bannon, the onetime adviser to former President Donald J. Trump.

“There are people who would cut Social Security — throw our grandparents into poverty — why?” Mr. Vance said. “So that one of Zelensky’s ministers can buy a bigger yacht?”

That prompted a public rebuke this month from a Republican colleague, Senator Thom Tillis of North Carolina, who ridiculed those who repeat unproven allegations.

“They’ve heard somebody say that if we pass this bill, that we’re all going to go ride to Kyiv with buckets full of money and let oligarchs buy yachts!” he said of critics of the assistance to Ukraine, in what he later called a reference to Mr. Vance’s comments. “I wonder how the spouses of the estimated 25,000 soldiers in Ukraine who have died feel about that? I mean, really, guys?”

Karoun Demirjian contributed reporting.