Kory Langhofer, counsel to Trump for America (TFA), said in a letter on Saturday to several congressional panels that Mueller's team obtained these emails from the General Services Administration (GSA), the government agency that hosted the transition email system, reports Xinhua news agency.
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.
The letter from Langhofer, who was counsel to Trump for America, alleged that career employees of the GSA improperly provided privileged communications to investigators working for Mueller. According to Langhofer, the assurance was made by then-GSA General Counsel Richard Beckler, who was hospitalized in August and has since died.
But late Saturday, another GSA official present for the conversation told Buzzfeed News that there was nothing improper about the disclosure of the emails to Mueller's team.
The president's legal team has said it has been cooperating with Mueller even as the president's lawyers continue to send signals that they expect the special counsel to begin wrapping up his probe soon.
Mr Loewentritt said Mr Beckler did not make a commitment to the transition team that requests from law enforcement for materials would be routed through transition lawyers.
Trump's transition team used facilities of the GSA, which helps manage the U.S. government bureaucracy, in the period between the Republican's November presidential election victory and his inauguration in January.
Transition officials signed agreements that warn them that materials kept on the government servers are subject to monitoring and auditing and there is no expectation of privacy, he added.
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Ty Cobb, a White House lawyer, said in November that he expected interviews with White House staff to wrap up shortly after Thanksgiving and that the vast majority of documents requested from the White House by Mueller were handed over in October.
Mueller's investigation is looking at whether any crimes were committed as part of what USA intelligence agencies say was systematic Russian meddling in the election Trump won. Trump's former national security adviser Michael Flynn pleaded guilty to lying to Federal Bureau of Investigation agents and agreed to be a cooperating witness in Mueller's probe.
Flynn was sacked by Trump in February for misleading senior administration officials about his contacts with Russia's ambassador to the U.S. But the sources said that was for naught, since Mueller has the complete cache from the dozen accounts.
Flynn attorney, Robert Kelner, declined to comment.
John Dowd and Jay Sekulow, who represent Trump personally, will likely use the meeting to get a sense of the next steps in Mueller's probe, which has already resulted in indictments and plea deals involving four former associates of the president.
The GSA did not respond immediately to a request for comment.
The media site Axios first reported on the transfer of the emails to Mueller's team.