President Trump’s legal team is setting the stage for its biggest showdown yet against special counsel Robert Mueller since the Russia probe began. Ahead of a highly anticipated meeting between Trump lawyers and special counsel investigators, an attorney for the President is demanding Mueller return transition documents believed to be unlawfully obtained.

Axios reports:

What’s new: A source close to Trump’s transition, which still exists as a legal entity so it can shut down what was once a 1,000-person operation, said the transition will send a letter to Mueller informing him that some of the emails are privileged, and asking for their return. The transition says it is willing to provide Mueller with vetted emails.

The source told me: “What they did is totally illegal, and they need to fix it.” But Peter Carr, spokesman for the Special Counsel’s Office, told Axios early this morning: “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.”

According to Axios boss Mike Allen, special counsel Robert Mueller secretly got his hands on “many tens of thousands,” of emails exchanged by Trump transition officials. Trump lawyer Kory Langhofer penned a blistering response to the news, accusing Mueller of unlawfully obtaining transition documents.

BREAKING NEWS: Trump lawyer: Mueller improperly obtained transition documents in Russia probe https://t.co/cCOzJX3OV1 pic.twitter.com/IqhdlNTqNw — Fox News (@FoxNews) December 16, 2017

Mueller obtained the emails through the General Services Administration (GSA) that were sent using Presidential Transition Team addresses ending in “ptt.gov.”

GSA “did not own or control the records in question,” Trump for America lawyer Kory Langhofer wrote to Mueller.

Langhofer noted the special counsel’s office has “extensively used the materials in question, including portions that are susceptible to claims of privilege.”

Trump transition officials only learned Mueller secretly obtained the emails after noticing special counsel investigators asking detailed questions based on the exchanges.

Trump transition officials were reportedly surprised by the move, as they have fully cooperated with the special counsel’s office since the probe began.

According to Allen, the special counsel’s office will use the emails to “generate new leads.”