A lawyer for President Trump's transition team said that special counsel Robert Mueller obtaining emails related to the transition was inappropriate, both on Mueller's behalf and the agency that provided them.

Fox News reported Saturday that Kory Langhofer, the counsel to the Trump for America transition team, wrote in a letter to Congress that Mueller “extensively used the materials in question, including portions that are susceptible to claims of privilege.”

Mueller's team, which is investigating Russia's interference in the 2016 election, reportedly obtained thousands of emails from transition team officials and has been using them to question the officials and build new leads in his probe, which is also looking at alleged ties between Trump's campaign and Russia.

The General Services Administration agency, which acted as the email server for the transition team, reportedly provided the emails to Mueller's team.

Langhofer alleges that the agency engaged in "unlawful conduct" by providing the emails, according to Fox.