A top executive at the tabloid publisher behind the National Enquirer said in a private email that he was “saving for my tombstone” the untold story of how the tabloid uncovered a 2019 exclusive about Jeff Bezos’s extramarital relationship, according to a lawsuit against the publisher.

The claim raises new questions about how American Media Inc (AMI) discovered the Amazon CEO’s relationship, and how it obtained knowledge of explicit sexual photographs that Bezos, one of the world’s richest men, has alleged were used against him by the publisher for “extortion and blackmail”.

The claim by Bezos that he was being blackmailed by AMI is the subject of an FBI investigation.

AMI has publicly insisted that it relied on only one source for its salacious scoop about the Bezos affair: Michael Sanchez, who is the brother of Lauren Sanchez, Bezos’s girlfriend.

Michael Sanchez was reportedly paid $200,000 by the tabloid for intimate texts and other information about the secret affair, which ultimately led to Bezos’s multibillion-dollar divorce from his wife, MacKenzie Bezos.

But in a new lawsuit filed in March against AMI, the Enquirer and top AMI executives, Michael Sanchez has accused the company of an elaborate plot to “scapegoat” him, and has cast doubt on the claim that he was the “sole source” of all the information and materials the tabloid obtained before it published its stories about the extramarital relationship.

Instead, Sanchez has claimed in court documents that the Enquirer publisher relied on multiple sources, including the use of “high-tech spyware” to secretly hack Bezos’s phone and extract “his most private and confidential information”.

Sanchez’s allegations echo claims that have been levelled against AMI by Bezos’s own security team. In a Daily Beast op-ed last year, Gavin De Becker, Bezos’s top security consultant, said he believed that Michael Sanchez had played a relatively minor role in the Enquirer’s story, and compared him to a “low-level Watergate burglar”.

“Reality is complicated, and can’t always be boiled down to a simple narrative like ‘the brother did it,’ even when that brother is a person who certainly supplied some information to a supermarket tabloid,” De Becker wrote.

It is still far from clear how the supermarket tabloid landed its scoop, and how it obtained information about some “below the belt” photographs Bezos sent to Lauren Sanchez while their relationship was still secret.

The Guardian reported in January that Bezos, the owner of the Washington Post, had his mobile phone “hacked” in 2018 after receiving a WhatsApp message that had apparently been sent from the personal account of the crown prince of Saudi Arabia, Mohammed bin Salman.

The matter is under investigation by experts at the United Nations. Saudi Arabia has denied it hacked Bezos.

In his lawsuit, Michael Sanchez admits having played a role in the Enquirer story, and claims to have done so “strategically” to minimise fallout for his sister and control the “narrative” of the story. While he has not denied supplying the tabloid with some of the couple’s private texts , he has denied the allegation that he ever gave AMI sexually explicit photographs.

Those photographs have never been published, but Bezos referred to them in an extraordinary blogpost statement on the matter in February 2019, a few weeks after the Enquirer story was published. Bezos claimed in the post that he was the victim of an extortion attempt by AMI.

The billionaire executive also published emails from AMI executives which he claimed showed the company threatening to publish sexually explicit photographs – which it described in detail – if Bezos did not publicly state that he had “no knowledge or basis for suggesting that AMI’s coverage was politically motivated or influenced by political forces”.

At the time Bezos said he had decided to publish the AMI emails “despite the personal cost and embarrassment they threaten”.

In his lawsuit, Michael Sanchez described himself as his sister’s “primary confidant and advisor” when he learned about her relationships with Bezos in March 2018. He said he was introduced to Bezos a month later over dinner, and said he had immediately “hit it off” with the billionaire.

A preliminary investigation that was commissioned by the Amazon CEO found that it was “highly probable” that an intrusion into Bezos’s mobile phone was triggered after an infected video file was sent to him over WhatsApp from the account of the Saudi crown prince on 1 May 2018.

In his lawsuit, Sanchez has alleged that he was first contacted by the Enquirer about his sister’s affair with Bezos in July 2018, and has claimed “on information and belief” that AMI was “already in possession of raunchy text messages and nude selfies exchanged” between Lauren Sanchez and Bezos.

Sanchez also referred in his lawsuit to a cryptic email he said was sent by Dylan Howard, AMI’s vice-president, dated 22 January 2019. A person close to the matter said it was a response to a previous email from Sanchez about the Bezos story, which had been published a few weeks earlier.

“The untold story – if you will – has not been told as to how we uncovered the story. I’m saving it for my tombstone,” Howard wrote in the email, according to Sanchez’s legal filing.

In a statement, AMI denied Sanchez’s allegations. “The fact, as we have maintained throughout, is that Mr Sanchez sold the National Enquirer the story about his sister’s secret affair and was the sole source for its reporting. His frivolous lawsuit underscores what his true motivation is, his own greed,” the company said.

AMI did not respond to multiple requests for comment about Howard’s email or what the executive was referring to.

An attorney representing Bezos did not respond to a request for comment.