Then-GSA General Counsel Richard Beckler in June explained to him that all requests for the transition's communications would indeed be routed through its lawyers and that GSA would hold all of the material in secure facilities. Numerous emails that Mueller's investigators have now include national security discussions about possible Trump global aims as well as candid assessments of candidates for top government posts, said those familiar with the transition.
Langhofer wrote that Mueller's office "received from the GSA tens of thousands of emails, including a very significant volume of privileged material", according to a copy of the letter published by Politico.
Mr Flynn was sacked by Mr Trump in February for misleading senior administration officials about his contacts with Russia's ambassador to the US. Van Grack also declined to identify the 13 transition officials whose materials the special counsel's office had obtained.
Treasury Secretary Steven Mnuchin told CNN: "I haven't heard anything about any firing, but we have to get past this investigation".
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.
Beckler died in September, and career GSA staff didn't consult or inform his successor of what Langhofer called "the unauthorized production of PTT materials". Trump campaign adviser George Papadoupolos and former National Security Adviser Michael Flynn, both for lying to the Federal Bureau of Investigation about their conversations with Russians.
But in his letter Langhofer said that Mueller's team has been "actively using" the emails obtained from GSA as part of the ongoing investigation.
"Democratic lawmaker Eric Swalwell was among the first to hit back at the claims: "'Private documents' on a U.S. Government, public email system?
Rep. Elijah Cummings, the top Democrat on the House Oversight and Government Reform Committee, questioned why Trump officials would be taking issue with an attempt by GSA to cooperate with investigators.
"The reality is that this administration has complied in every single possible way with the special counsel, tax payers have spent millions and millions of dollars on this investigation that has not yet proven any sense of collusion with the Russians", Short said.
"Of course Mueller obtained emails from a third party", Renato Mariotti, a former federal prosecutor, said, also on Twitter.