US special counsel Robert Mueller’s team believes a rightwing author and conspiracy theorist tipped off Roger Stone, a Donald Trump ally, months before WikiLeaks released thousands of emails stolen from Hillary Clinton’s campaign chair, according to a draft legal document.

The document, part of a plea offer to Jerome Corsi, provides unprecedented insight into an active part of Mueller’s investigation into Russian election interference and possible coordination with Trump’s associates.

It reveals Mueller is keenly focused on whether Americans close to the Trump campaign knew about WikiLeaks’ plans to release hacked material during the 2016 presidential campaign.

The document’s contents were first reported by NBC News, and a copy of it was posted online by the Washington Post. Corsi told the Associated Press on Tuesday evening that the document had been provided to his attorney by Mueller’s team.

Stone has denied having prior knowledge of WikiLeaks’ plans. Peter Carr, a spokesman for Mueller, did not immediately return a message seeking comment.

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Corsi said the document, similar to others filed by Mueller in previous plea deals, contains portions of emails he exchanged with Stone in summer 2016 about WikiLeaks. But he denied he intentionally lied to investigators about the emails, and said he rejected the plea offer because he would have been charged with one count of making false statements.

“I did not ever wilfully and knowingly give them false information,” Corsi said, adding that he forgot about the emails in question during his first interview with Mueller’s team. The emails were among 60,000 stored on the laptop he provided to the special counsel’s office.

According to the document, the emails were exchanged in late July and early August 2016, more than two months before WikiLeaks published thousands of emails stolen from the private email account of Clinton campaign chair John Podesta.

In late July 2016, the document shows, Stone emailed Corsi asking him to get in touch with Julian Assange, the WikiLeaks founder who has been living in Ecuador’s embassy in London since 2012. Stone said he wanted Corsi to try to obtain emails the group possessed about Clinton.

The document says Corsi passed Stone’s request to an “overseas individual”, whom Corsi identified as Ted Malloch, a London-based academic who has said he was also questioned by Mueller.

The document quotes Corsi’s response to Stone on 2 August 2016. “Word is friend in embassy plans 2 more dumps. One shortly after I’m back. 2nd in Oct. Impact planned to be very damaging,” wrote Corsi, who was in Europe at the time. He then told Stone it was time for Podesta to “be exposed as in bed w enemy if they are not ready to drop HRC,” a reference to Clinton.

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On Tuesday, Corsi told the AP that the email he sent Stone – which accurately forecast that WikiLeaks would release derogatory information about Podesta in October – was based on his own deduction and not the result of any inside information or a source close to the group.

“It’s all a guess. That email – ‘word is’ – is 100% speculation on my part, a package so that Roger’s not going to dismiss it because I’m real sure I’m right,” he said.

He said he had never had contact with Assange and that he did not obtain any advance knowledge of WikiLeaks’ plans.

Corsi said he told investigators – including Mueller team members Jeannie Rhee, Aaron Zelinsky and Andrew Goldstein – that he had told Stone that Assange had the Podesta emails. “But I maintained and still do that I figured it out,” he said, adding: “I made it sound maybe like I had a source, but I didn’t. And I don’t think Stone ever believed me.”

Corsi said prosecutors would not believe him, thinking he was trying to protect Stone. And he believes he was threatened with a felony charge “because I couldn’t give them what they wanted” by connecting Stone to WikiLeaks.

Corsi said he also believed the plea offer was extended to prevent him from speaking publicly about his contact with Mueller’s team. He said he did not know if Mueller would follow up with charges.

The last time his attorney, David Gray, heard from Mueller’s team was on Monday after he had publicly rejected the deal. According to Corsi, they told Gray: “We’ll take it from here.”