Open this photo in gallery The Enquirer published a story last month that included racy texts between Bezos, seen above, and former TV anchor Lauren Sanchez. The Associated Press

Private investigators working for Jeff Bezos have concluded that the brother of the Amazon chief executive’s mistress leaked the couple’s intimate text messages to the National Enquirer, a person familiar with the matter told the Associated Press on Monday.

The person wasn’t authorized to discuss the matter publicly and spoke to AP on condition of anonymity.

The findings add to the intrigue surrounding the clash between the pro-Trump tabloid and the world’s richest man. Mr. Bezos’s investigators have suggested the Enquirer’s coverage of his affair was driven by dirty politics. U.S. President Donald Trump has been highly critical of Mr. Bezos over his ownership of The Washington Post and Amazon, and the Post’s coverage of the White House.

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The brother, Michael Sanchez, is a supporter of Mr. Trump and an acquaintance of Trump allies Roger Stone and Carter Page. He is also the manager of his sister, Lauren Sanchez, a former TV anchor. The investigators have not said how they believe Mr. Sanchez came into possession of his sister’s intimate messages.

Mr. Sanchez did not immediately respond to a message seeking comment on Monday. In a Jan. 31 tweet, he said without evidence that Mr. Bezos’s long-time security consultant, Gavin de Becker, who is leading the private investigation, “spreads fake, unhinged conservative conspiracy theories.”

An attorney for the tabloid’s parent company did not immediately respond to an e-mail seeking comment.

On Sunday, an attorney for the head of American Media Inc. (AMI), which owns the Enquirer, said that the information for the story had been provided by a “reliable source” well-known to Mr. Bezos and Ms. Sanchez. The source had provided information to the company for at least seven years, Elkan Abromowitz, an attorney for AMI chief executive David Pecker, said on ABC’s This Week.

When asked if Mr. Sanchez was the source, Mr. Abromowitz said: “I’m not permitted to tell you or confirm or deny who the source is.”

But the Daily Beast, citing people inside AMI, reported that Mr. Sanchez was the Enquirer’s source.

Mr. Bezos ordered the investigation after the Enquirer published a story about the affair last month. The investigators have since turned over the results of their probe to attorney Richard Ben-Veniste for review and possible referral to law enforcement. Mr. Ben-Veniste had served as special prosecutor during the Watergate scandal.

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Mr. Bezos has said AMI threatened to publish explicit photos of him unless he stopped investigating how the Enquirer obtained his private exchanges, and publicly declared that the Enquirer’s coverage of him was not politically motivated.

Federal prosecutors are also looking into whether the Enquirer violated a co-operation and non-prosecution agreement that recently spared the tabloid and top executives from charges for paying hush money to a Playboy model who claimed she had an affair with Mr. Trump, two people familiar with the matter said. The people weren’t authorized to discuss the matter and spoke on condition of anonymity.