Robert Mueller has obtained thousands of emails from 12 accounts linked to Donald Trump's transition team.
The special counsel investigating alleged collusion with Russia is using the cache to confirm existing information and open up new leads, according to Axios.
Kory Langhofer, a lawyer who represented Mr Trump’s campaign and worked for the President-elect’s transition team, has claimed the messages were obtained through unlawful means.
In a letter to the House and Senate oversight committees, Mr Kanghofer wrote that Mr Mueller’s team had unlawfully obtained the documents from the General Services Administration (GSA).
Career staff members at the agency “unlawfully produced [Trump for America’s] private materials, including privileged communications, to the special counsel’s office,” according to the letter, a copy of which was seen by Reuters. It said the materials included “tens of thousands of emails”.
The transition team communicated via GSA email addresses in the period between Mr Trump’s election in November and his inauguration in January.
Mr Mueller is a former director of the FBI and a seasoned prosecutor.
Mr Langhofer asked Congress to “act immediately to protect future presidential transitions from having their private records misappropriated by government agencies,” according to reports.
The transition team attorney confirmed to The Independent that these reports were accurate, but declined to provide a copy of his letter. Mr Langhofer also declined to say why he believed the disclosure had been unlawful.
The special counsel’s office declined to comment to The Independent.
White House spokeswoman Sarah Huckabee Sanders said the administration would “continue to cooperate fully with the special counsel and expects this process to wrap up soon”.
Mr Mueller is investigating allegations that the Trump campaign colluded with Russia in its bid for the White House. He has so far charged four former campaign officials with crimes ranging from tax fraud to lying to the FBI. Two officials – former national security adviser Michael Flynn and campaign adviser George Papadopoulos – have pleaded guilty.
Mr Trump has dismissed the special counsel’s investigation as a politically motivated “witch hunt”. ”There is absolutely no collusion. That has been proven,” he told reporters on Friday. Russia denies interfering in the election.
His legal team has suggested appointing a second special counsel to investigate Mr Mueller and his team for what they see as conflicts of interest. One of the President’s attorney’s, Ty Cobb, told the Washington Post he believes Mr Mueller’s investigation of the White House will be over by the end of the year.
Mr Trump’s personal lawyers are slated to meet with Mr Mueller as early as next week, according to CNN.
Additional reporting by agencies