WASHINGTON (Reuters) - U.S. Special Counsel Robert Mueller has issued a grand jury subpoena to another associate of longtime Donald Trump adviser Roger Stone, a lawyer for Andrew Miller, the subject of the subpoena, said on Thursday.

FILE PHOTO: U.S. political consultant Roger Stone, a longtime ally of President Donald Trump, speaks to reporters after appearing before a closed House Intelligence Committee hearing investigating Russian interference in the 2016 U.S. presidential election at the U.S. Capitol in Washington, U.S., September 26, 2017. REUTERS/Kevin Lamarque

The subpoena is the latest to suggest that Mueller, who is probing Russian meddling in the 2016 U.S. presidential election, is focusing in part on Stone and whether he might have had advance knowledge of material allegedly hacked by Russian intelligence.

Paul Kamenar, Miller’s lawyer, said his client, who normally works as a house painter, would be flying to Washington and was likely to appear at the federal courthouse there on Friday morning. The grand jury hearing testimony for Mueller’s investigation regularly meets at the courthouse.

Kamenar said he would be filing a motion with the court on Thursday seeking to have Mueller’s subpoena quashed on the grounds that Mueller’s appointment is legally unsound and illegitimate. He said he expected the court to seal this motion.

A Russian company indicted by Mueller for allegedly seeking to interfere in U.S. politics, Concord Management and Consulting, has made similar court arguments challenging Mueller’s authority.

Kamenar said Miller had already turned over some documents to Mueller’s team but that Mueller’s office was pushing for further document disclosures. Mueller’s subpoena to Miller was first reported by the New York Times.

Miller is the third associate of Stone to be identified as having been contacted by Mueller’s investigators. Jason Sullivan, a social media expert from California hired by Stone during the 2016 election campaign, recently flew to Washington to appear before Mueller’s grand jury.

John Kakanis, who worked for Stone as an assistant and driver, has also been subpoenaed to give testimony to Mueller, Reuters reported last month.

Stone, an early Trump backer whose reputation as an aggressive political operative dates back to the Watergate scandal of the 1970s, has said that eight of his associates have been contacted by Mueller.

However, while he has appeared before the House Intelligence Committee, Stone has said that he has not been approached in any way by investigators for Mueller. Stone did not immediately respond to a request for comment on the Miller subpoena.