(Reuters) - An author and conspiracy theorist who communicated with a longtime adviser to U.S. President Donald Trump has been subpoenaed as part of Special Counsel Robert Mueller’s investigation of Russian interference in the 2016 U.S. election, the author’s lawyer said on Wednesday.

Jerome Corsi, a contributor to conspiracy theory websites, is the latest among associates of Republican political adviser Roger Stone called to testify to a grand jury or be questioned by Mueller’s prosecutors.

In recent months, Stone has been the subject of scrutiny by Mueller’s office. Investigators appear focused on Stone’s contacts with WikiLeaks and its founder Julian Assange, according to sources familiar with the interviews of the associates.

Before the election, WikiLeaks published emails hacked from the Democratic Party and the personal email account of John Podesta, candidate Hillary Clinton’s campaign chairman.

Mueller’s office is looking into whether Stone had advance knowledge of material detrimental to Clinton that U.S. intelligence agencies have concluded was obtained by Russia through hacking, the sources familiar said. Moscow has denied allegations it meddled in the election campaign.

“We intend to cooperate with the subpoena,” said Corsi’s lawyer David Gray. He said Corsi plans to appear on Friday before the grand jury convened by Mueller’s prosecutors.

“We are assuming this is about Roger Stone,” Gray said. Gray said Corsi and Stone communicated between 2016 and 2018 and they planned to provide the computer and phone records to investigators.

Stone did not respond to a request for comment on Wednesday.

The subpoena was first reported by the New York Times.

Peter Carr, a spokesman for Mueller, declined to comment.

Corsi helped promote the “birther” movement questioning President Barack Obama’s U.S. citizenship. In 2017 he was named the Washington bureau chief for Infowars, a website run by conspiracy theorist Alex Jones. Corsi no longer works there.

Stone has said that he has neither been interviewed by Mueller’s team nor summoned before a grand jury. Stone has also told reporters that he expects to be indicted but without making clear why.

Stone, who first earned a reputation as a young pro-Nixon operative during the Watergate scandal, has said he knows of eight associates who so far have been approached by Mueller’s investigators.

Four people familiar with recent contacts by Mueller’s team to associates of Stone told Reuters that prosecutors have shown them emails and phone messages to or from Stone and questioned them about their meaning.

“Roger must have told you stuff,” one associate who testified in front of the grand jury said prosecutors insisted to him. Prosecutors pressed him on whether Stone had spoken with Assange, something the associate said he could not confirm.

The U.S. Senate intelligence panel, which is among several congressional committees investigating the Russia issue, is also looking into the role of Wikileaks, which publishes secret information.

WikiLeaks founder Assange, an Australian, is in self-exile with diplomatic immunity at the Ecuadorean Embassy in London. He took refuge there six years ago while being pursued by British authorities for possible extradition to Sweden in a since-dropped sexual molestation case.