Trump's legal team has reacted with outraged claims of unconstitutional and illegal tactics, while Fox News hosts have begun wildly accusing the Federal Bureau of Investigation and Mueller of a "coup".
The transition sources said they were surprised about the emails because they have been in touch with Mueller's team and have cooperated.
Why it matters: The transition emails are said to include sensitive exchanges on matters that include potential appointments, gossip about the views of particular senators involved in the confirmation process, speculation about vulnerabilities of Trump nominees, strategizing about press statements, and policy planning on everything from war to taxes.
Langhofer also said that a GSA official appointed by Trump in May had assured the transition in June that any request for records from Mueller's office would be referred to the transition's attorneys. Beckler died in September. Transition officials signed agreements that warn them that materials kept on the government servers are subject to monitoring and auditing, he told Buzzfeed, and there's no expectation of privacy.
A lawyer for TFA, who spoke on background because he was not authorized to speak to media, told NPR it found out on December 13 that GSA officials turned over the emails to the Special Counsel in September.
This dispute over the lawful obtaining of this batch of emails is the latest in a series of ongoing attacks, particularly from conservatives, on the Department of Justice investigation looking into what role Russian Federation played in interfering in the 2016 presidential election.
There are rumors around Washington suggesting Trump's firing of Mueller is imminent.
A spokesman for the special counsel said in a statement that the emails were obtained properly.
"GOP voices are accusing the team assembled by special counsel Robert Mueller of bias against President Trump - and they're appearing to set the stage for some action".
Marc Short, Trump's director of legislative affairs, was asked on NBC if Trump was laying the groundwork for firing Mueller and responded: "There's no conversation about that whatsoever in the White House". But he has apparently caught Michael Flynn in a lie about his lobbying activities and may have former Trump campaign manager Steve Bannon on money laundering crimes he committed before he even knew the president.
On whether Mueller should have disclosed that they had obtained the emails, Loewentritt added, "That's between the Special Counsel and the transition team".
The documents were provided to Mueller's team by the GSA in September in response to requests from the Federal Bureau of Investigation, but the transition wasn't informed at the time, according to people familiar with the transition organization.