Special counsel Robert Mueller's office on Sunday defended its work after a lawyer for President Trump's transition team accused investigators of improperly obtaining thousands of emails from transition officials.

"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," Peter Carr, a spokesman for the special counsel's office, said in a statement to The Hill.

Axios reported Saturday that Mueller's team is now in possession of tens of thousands of emails from the Trump transition team, including messages from Trump's son-in-law and senior adviser Jared Kushner, as well as other members of the transition's political leadership and foreign policy team.

Mueller's prosecutors reportedly used the emails to question witnesses, and are also looking to the messages to confirm information and follow new leads.

Axios reported that the special counsel obtained the emails from the General Services Administration, which managed the transition team's email accounts.

But in a letter to several members of Congress, a lawyer for Trump's transition team, Trump for America (TFA), said that Mueller obtained the emails illegally.

Kory Langhofer accuses the GSA of "unlawfully produc[ing] TFA's private materials, including privileged communications, to the Special Counsel's Office," according to Reuters.

Langhofer's letter, which was sent to the Senate Homeland Security and Governmental Affairs Committee and the House Oversight and Government Reform Committee, calls on the committees "to protect future presidential transitions from having their private records misappropriated by government agencies, particularly in the context of sensitive investigations intersecting with political motives."

Mueller's investigation into Russian interference in the 2016 election has heated up in recent weeks after Trump's former national security adviser Michael Flynn pleaded guilty to lying to the FBI about his contacts with Russians.

The investigation has also seen Trump's former campaign chairman, Paul Manafort, indicted on multiple charges.