Nunberg told The Washington Post he was asked to appear before a grand jury on Friday and that Mueller also subpoenaed documents related to Trump and nine other individuals who worked on the campaign or in the White House.
Earlier, media reported that Robert Mueller's team sought to receive emails, text messages, working papers, and information about telephone calls dating back to 2015.
21 February 2018, 16:52 GMT
Also the team requested the information on nine former and current members of Trump's administration, including Steve Bannon, the former senior counselor to the US president, Trump's lawyer Michael Cohen, campaign head Paul Manafort, and campaign deputy Richard Gates, according to the NBC broadcaster, that obtained a copy of the subpoena.
Under the Mueller's guidance, the FBI and the Justice Department have been investigating alleged links between the Trump campaign and Moscow. The US President and Russian officials have repeatedly denied the claims of interference, pointing out that no evidence has been offered to substantiate the allegations.