The world's richest man, Amazon founder Jeff Bezos, has accused the owner of a US gossip magazine of trying to blackmail him over private pictures.
He said the National Enquirer's parent company, American Media Inc (AMI), wanted him to stop investigating how they had obtained his private messages.
Hours after Mr Bezos announced his divorce last month, the magazine published details, including private messages, of an extramarital affair.
AMI says the company "acted lawfully".
"American Media believes fervently that it acted lawfully in the reporting of the story of Mr Bezos. Further, at the time of the recent allegations made by Mr Bezos, it was in good faith negotiations to resolve all matters with him," the company said in a statement.
"Nonetheless, in light of the nature of the allegations published by Mr Bezos, the Board has convened and determined that it should promptly and thoroughly investigate the claims. Upon completion of that investigation, the Board will take whatever appropriate action is necessary."
What did Bezos say?
In a stunning blog post on Thursday, Mr Bezos posted an email he said had been sent to his intermediaries by AMI's representatives threatening to publish "intimate photos" of him and his lover, former TV host Lauren Sanchez.
The billionaire, who also owns the Washington Post newspaper, said AMI had wanted him to make a "false public statement" that the National Enquirer's coverage of him and his mistress was not politically motivated.
According to emails included by Mr Bezos in his blog, an AMI lawyer proposed on Wednesday that the photos would not be published in return for a public statement "affirming that [Bezos and his team] have no knowledge or basis" to suspect such a motive.
It comes after Mr Bezos' investigator suggested they had "strong leads" to suspect political reasons.
"Rather than capitulate to extortion and blackmail," wrote Mr Bezos, "I've decided to publish exactly what they sent me, despite the personal cost and embarrassment they threaten."
Early in the blog post, Mr Bezos mentions AMI's links to President Donald Trump.
What has AMI said?
Reacting to the allegations on Friday, AMI said the company "believes fervently that it acted lawfully in the reporting of the story of Mr Bezos".
AMI said that they had been "in good faith negotiations to resolve all matters with him" when the allegations were made, and that the board "has convened and determined that it should promptly and thoroughly investigate the claims".
Why does Bezos mention Trump?
Mr Bezos said his ownership of the Washington Post was a "complexifier" for him because he had made enemies of "certain powerful people", including President Trump, who is a friend of AMI's boss, David Pecker.
AMI recently admitted in court that it had co-ordinated with the Trump presidential campaign to pay a Playboy model $150,000 (£115,000) in hush money to keep quiet about her alleged affair with Mr Trump.
Mr Bezos notes in his blog post how the publisher had confessed to the so-called "catch and kill" deal to bury Karen McDougal's politically embarrassing story.
AMI's agreement to co-operate with federal authorities means it will not face criminal charges over the payments, Manhattan prosecutors announced in December.
Mr Trump's former lawyer Michael Cohen - who facilitated the hush money at the direction, he says, of Mr Trump - has already admitted violating campaign finance laws.
What about Bezos' reputation?
The Amazon boss did not try to hide the potential for embarrassment, writing "of course I don't want personal photos published" and noting what he called "AMI's long-earned reputation for weaponising journalistic privileges".
"But," he continued, "I also won't participate in their well-known practice of blackmail, political favours, political attacks, and corruption.
"I prefer to stand up, roll this log over, and see what crawls out."
His blog contained itemised details of 10 pictures in an email from the magazine's editor, Dylan Howard, who said they had been "obtained during our newsgathering".
New Yorker writer Ronan Farrow tweeted that he "and at least one other prominent journalist" had been subject to similar "stop digging or we'll ruin you" threats from AMI.
Mr Bezos said "AMI's claim of newsworthiness is that the photos are necessary to show Amazon shareholders that my business judgment is terrible".
But the Amazon boss countered that the firm's results "speak for themselves".
Dylan Howard's name, along with those of two National Enquirer reporters, appeared on an 11-page story the magazine published on 9 January containing alleged details of Mr Bezos' affair with Ms Sanchez.
The tabloid labelled it "the biggest investigation in Enquirer history!".
How did they get the images and messages?
This has been the focus of Mr Bezos's private investigators.
The probe has been led by security Gavin de Becker - who is the billionaire's long-term head of security.
He has said that "strong leads point to political motives" over the leak, but has not divulged much further information.
Mr Sanchez, a publicist who lives in West Hollywood, has strongly rejected involvement.
"I am not dignifying de Becker's passive aggressive allegations or his crazy conspiracy theories," he said in a statement to Page Six. He also said he had recommended his sister fire Mr de Becker.
US media report that Mr Sanchez knows AMI's David Pecker - as well as other people with links to Mr Trump including his former campaign aides Carter Page and Roger Stone.
He has confirmed to the Daily Beast that he knows Mr Sanchez - describing him as a "very good guy".