The special counsel, Robert Mueller, investigating allegations of Russian meddling in the US 2016 election had obtained tens of thousands of emails unlawfully.
Mueller is an American lawyer and civil servant. He was the sixth Director of the Federal Bureau of Investigation from 2001 to 2013.
An organisation established for US President Donald Trump’s transition to the White House revealed the information on Saturday.
Kory Langhofer, counsel to the transition team known as Trump for America, wrote a letter to congressional committees.
The letter says Special Counsel Robert Mueller’s team had improperly received the emails from the General Services Administration (GSA), a government agency.
It also says the materials included tens of thousands of emails adding that "career staff members at the agency unlawfully produced TFA’s private materials, including privileged communications, to the Special Counsel’s Office,"
The Trump team’s accusation adds to the growing friction between the president’s supporters and Mueller’s office.
The reasons of growing friction are it investigates whether Russia interfered in the election and if Trump or anyone on his team colluded with Moscow.
Asked for comment, White House spokeswoman Sarah Sanders said: “We continue to cooperate fully with the special counsel and expect this process to wrap up soon.”
The special counsel’s office waved off the transition team’s complaint.
“When we have obtained emails in the course of our ongoing criminal investigation, we have secured either the account owner’s consent or appropriate criminal process,” said Peter Carr, spokesman for the special counsel’s office.
The GSA did not respond immediately to a request for comment.