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"I've been really worried recently about the whole Russian situation," wrote the prankster, who tweets under the name @SINON_REBORN and provided the emails to Business Insider. "The White House will be okay won't it? I love my job, and the people I work with, I don't want the dream to end up derailing."
Cobb said in response: "I have great confidence there is nothing there implicating the President or the White House. Manafort and Flynn have issues separate and apart from the WH that will cause the investigation to linger but am hoping we get a clean bill of health soon. Best, Ty."
Manafort's spokesman declined to comment. Flynn's lawyer did not respond to a request for comment.
Asked to comment on the exchange, Cobb told Business Insider: "No idea--it is a felony of course to impersonate a government official of course, or to conspire to." He did not respond to follow-up questions.
Cobb's assertion that "there is nothing there implicating the President or the White House" comes amid reports that FBI special counsel Robert Mueller is building an obstruction-of-justice case against the president over his firing of former FBI Director James Comey.
Mueller will also be interviewing aides who were aboard Air Force One when the president is said to have drafted a misleading statement on behalf of his son Donald Trump Jr. about a meeting he had with two Russians at Trump Tower last June, according to CNN.
Cobb, who was hired in July to manage the legal and media response to the Russia investigation, may have offered a window into the White House's assessment of Manafort and Flynn's standing in the ongoing probe.
Thomson Reuters Paul Manafort
"This isn't far off from a reasonable legal analysis based on what we know publicly," said former federal prosecutor Renato Mariotti. "We know Manafort has serious potential liability given the search warrant executed at his home, but that could be for something discrete like a false disclosure. Ditto with Flynn."
But Cobb's analysis is "interesting," Mariotti said, because "he presumably has interviewed all of the relevant witnesses and read the documents. ... He knows more than we know."
Manafort has emerged as a focal point of Mueller's probe into Russia's election interference and whether the Trump campaign colluded with Moscow.
Mueller recently recruited New York Attorney General Eric Schneiderman to help investigate the longtime political operative for possible financial crimes and money laundering. The IRS's criminal-investigations unit has been brought onto the investigation to examine similar issues, according to The Daily Beast, though it is unclear to what extent its work will focus on Manafort.
Mueller's team obtained a search warrant to raid Manafort's home in July.
Mueller's investigators are also reportedly looking into whether former Flynn took part in efforts to obtain emails from Russian hackers deleted from Hillary Clinton's private email server.
Flynn came under scrutiny earlier this year over his frequent contact with Russia's ambassador to the US during the election and his belated registration as a foreign agent for his lobbying work on behalf of a Turkish businessman last year.
Russian operatives also boasted during the 2016 election that they could use their relationship with Flynn to make inroads with Trump and his associates, CNN reported earlier this year.