Paul Manafort, President Trump's former campaign chairman, leaves a federal court hearing earlier this year. (Jose Luis Magana/AP)

It seemed to be another bombshell story amid all the revelations about the investigation of the Trump campaign’s ties to Russia.

The Guardian newspaper reported last week that Paul Manafort, President Trump’s former campaign chairman, had met with WikiLeaks founder Julian Assange several times, including during a critical period in March 2016.

The story suggested that the meeting in London could be a key link in special counsel Robert S. Mueller III’s investigation of the Trump campaign. If such a meeting occurred, it would establish the first direct contact between one of the president’s associates and WikiLeaks, which began releasing emails hacked from the Democratic National Committee in summer 2016. The emails, stolen by Russian agents and passed to WikiLeaks, proved damaging to Trump’s opponent, Democrat Hillary Clinton.

The Guardian reported that the alleged Manafort-Assange meeting “could shed new light” on the events leading up to the leaks and might indicate coordination among WikiLeaks, Trump’s campaign and Russian hackers. Trump has repeatedly denied any such collusion.

But one week after publication, the Guardian’s bombshell looks as though it could be a dud.

No other news organization has been able to corroborate the Guardian’s reporting to substantiate its central claim of a meeting. News organizations typically do such independent reporting to confirm important stories.

The Guardian, which is based in London but has a significant American presence, has stood by the story, albeit somewhat halfheartedly. It has said little to defend itself amid mounting criticism.

The lead reporter on the Manafort article, Luke Harding, declined to comment on Monday and referred questions to the newspaper’s spokesman, Brendan O’Grady.

In response to questions, O’Grady reissued the same statement the Guardian has stuck by for the past six days: “This story relied on a number of sources. We put these allegations to both Paul Manafort and Julian Assange’s representatives prior to publication. Neither responded to deny the visits taking place. We have since updated the story to reflect their denials.”

He declined to comment further.

However, the Guardian did tweak some of the language in its original report to sound less definitive in its conclusions. The original refers to “the meeting,” but that was updated to “the apparent meeting.”

Manafort and WikiLeaks have blasted the Guardian’s reporting. Manafort — who was convicted in August on eight counts of bank- and tax-fraud charges arising from his decades-long business and political activities in Ukraine — has called the article “totally false and deliberately libelous.” He said in a statement last week that he’s never met Assange.

A most colorful rebuke came from WikiLeaks, which tweeted last week, “Remember this day when the Guardian permitted a serial fabricator to totally destroy the paper’s reputation. @WikiLeaks is willing to bet the Guardian a million dollars and its editor’s head that Manafort never met Assange.”

WikiLeaks on Monday identified the alleged fabricator as Fernando Villavicencio, an Ecuadoran journalist and activist. A government ministry under Ecuador’s previous government accused Villavicencio of fabricating documents; Villavicencio’s supporters call him a crusading journalist who exposed corruption under former president Rafael Correa.

Villavicencio’s byline appears on the Guardian’s Manafort article, but only in the newspaper’s print edition, which doesn’t circulate widely outside Great Britain. O’Grady declined to explain why Villavicencio’s name was left off the Web version of the article, which was viewed around the world last week.

The Guardian and WikiLeaks have a long and tumultuous history. They began as allies, but the relationship — particularly between Harding and Assange — has soured over the years. The roots of their falling-out date to the Guardian’s collaboration with WikiLeaks on the release of secret American diplomatic and military documents beginning in 2010. Assange objected to Harding and co-author David Leigh’s subsequent book, “WikiLeaks: Inside Julian Assange’s War on Secrecy,” which Assange has called “a tour de force of dreary cash-in publishing.” He has branded Harding “a serial plagiarist.”

In a review for Newsweek magazine in 2015, Assange called “The Snowden Files” — Harding’s book about National Security Agency whistleblower Edward Snowden — “a hack job in the purest sense of the term. . . . It is a book by someone who wasn’t there, doesn’t know, doesn’t belong and doesn’t understand.”

Among the many people calling the Guardian’s Manafort-visits-Assange story into question is Glenn Greenwald, the journalist who won a Pulitzer Prize in 2014 for the Guardian’s U.S.-based website for reporting on Snowden and the National Security Agency.

“I can’t say definitively that it’s false because I’m not able to prove a negative,” said Greenwald, who left the Guardian in 2013 to start the Intercept news site. “But speaking for myself [as an editor], if you publish a story of this magnitude, you’d want to see a lot more evidence.”

The Guardian story relied on unnamed sources and an “internal document” written by Ecuador’s intelligence agency that lists “Paul Manaford [sic]” as one of several guests at the Ecuadoran Embassy in London, where Assange has sought refuge since 2012 to avoid extradition to Sweden for alleged sex crimes. The newspaper said the document also mentions that “Russians” were guests at the embassy, but the story didn’t identify any individuals, nor did it reproduce the document in question.

The newspaper said Manafort visited the embassy three times, the last time in March 2016 when he met with Assange for about 40 minutes. It reported that he was “casually dressed when he exited the embassy, wearing sandy-coloured chinos, a cardigan and a light-coloured shirt.”

But the story doesn’t specify the date of the alleged meeting. In addition, no photos or video of Manafort entering the embassy have emerged. The Guardian is silent about whether its reporters saw any such photographic evidence.

Greenwald notes that the embassy is surrounded by cameras that record who enters and leaves. “If Paul Manafort got anywhere near that building, let alone three times, there would be mountains of evidence” in the hands of Ecuadoran intelligence officials, whom the Guardian cited as the source of its story.

If the Guardian stands fully by its work, Greenwald said, it should welcome questions and reply to its critics with something more than a short statement.

“It’s so ironic that the institutions that demand disclosure from others are the least willing to be transparent about themselves,” he said.