Amazon.com Inc. Chief Executive Officer Jeff Bezos accused the National Enquirer and its publisher David Pecker of extortion and blackmail, stepping up a war of words between the world’s richest man and a confidant of U.S. President Donald Trump.

The magazine published an expose on Bezos’s relationship with former TV anchor Lauren Sanchez, and Bezos has hired investigators to find out if the story was politically motivated. He owns the Washington Post, which has written critical stories about Trump, who counts Pecker as a close ally.

In an unprecedented and surprising move on Thursday, Bezos said the National Enquirer threatened to publish more details and revealing photos if the executive didn’t stop the probe.

"Rather than capitulate to extortion and blackmail, I’ve decided to publish exactly what they sent me, despite the personal cost and embarrassment they threaten," Bezos wrote in a post on Medium. "Any personal embarrassment AMI could cause me takes a back seat because there’s a much more important matter involved here. If in my position I can’t stand up to this kind of extortion, how many people can?"

A spokesman for American Media Inc., publisher of the National Enquirer, didn’t immediately respond to a request for comment. The magazine investigated Bezos because his wealth and position made him a newsworthy subject, and its reporting isn’t influenced by politics, according to copies of emails allegedly from AMI lawyer Jon Fine that Bezos included in his post on Thursday.

In one of those emails, Fine proposed Bezos release a mutually agreed upon statement to a news outlet saying that he had no basis for suggesting AMI’s coverage was politically motivated. In exchange, the media group would refrain from publishing other texts and photos featuring Bezos, according to the email released by the Amazon CEO.

Fine, a former executive at Amazon, didn’t immediately respond to an email seeking comment. The White House also didn’t respond to a request for comment.

Bezos last month posted a statement on Twitter, signed by him and his wife of 25 years, MacKenzie, announcing their plans to divorce. Hours later, the first Enquirer story on his relationship with Sanchez posted online.

Bezos is Amazon’s largest shareholder, with a stake of about 16 percent. An Amazon spokesman has said the CEO and founder remains "focused and engaged in all aspects of Amazon." Bezos and company spokespeople haven’t addressed whether he will accelerate his periodic sales of Amazon shares. An Amazon spokesman declined to comment on Thursday.

Pecker is cooperating with federal prosecutors who are investigating so-called catch-and-kill payments the Enquirer made ahead of the 2016 presidential election to at least one woman who alleged she had an affair with Trump. The Enquirer secured the rights to the material, assuring the allegations wouldn’t be shared with other media, then never published the story.

American Media is backed by Anthony Melchiorre and his hedge fund, Chatham Asset Management. Years before Trump ascended to the White House, Melchiorre threw a financial lifeline to Pecker’s company and ended up with about an 80 percent stake.

Billionaire hedge fund manager Leon Cooperman also has put money into the publisher. He said last year that Chatham executives introduced him to the investment.