Mueller has interviewed the cybersecurity expert who said he was 'recruited to collude with the Russians'
Matt Tait, a former information security specialist at Britain's Government Communications Headquarters who tweets as @pwnallthethings, was interviewed by Mueller several weeks ago, according to a source familiar with the matter.
The interview was part of a broader effort by Mueller to examine the relationship between the longtime GOP operative - the late Peter Smith - and former national security adviser Michael Flynn, and whether Flynn played any role in seeking out the stolen emails during the election.
The House Intelligence Committee has also interviewed Tait in its probe into Russia's election interference, according to CNN.
Tait declined to comment. A spokesman for Mueller's office declined to comment.
Mueller interviewed Tait months after Tait published a first-person account of his interactions with Smith on the national security blog, LawFare.
In a piece titled "The Time I Got Recruited to Collude with the Russians," Tait recalled how Smith had approached him to help verify the authenticity of emails he said were given to him by dark web hackers.
"Smith implied that he was a well-connected Republican political operative," Tait wrote, and had contacted him because he believed "that Clinton's private email server had been hacked-in his view almost certainly both by the Russian government and likely by multiple other hackers too."
Tait said he went along with Smith's request because he wanted to find out more about these hackers - and whether they pranksters or part of a larger Russian intelligence front.
He said he never found out. But he wrote that it was "immediately apparent that Smith was both well connected within the top echelons of the [Trump] campaign and he seemed to know both Lt. Gen. [Michael] Flynn and his son well." Smith also listed top Trump advisers Kellyanne Conway, Steve Bannon, and Sam Clovis as part of a group he had formed to conduct opposition research on Clinton.
Tait's interaction with Smith went on for at least a few weeks, he wrote, and at one point he sent Tait a document detailing the company Smith and his colleagues had set up "as a vehicle to conduct" the opposition research.
The company, KLS Research, had been set up as a limited-liability corporation in Delaware "to avoid campaign reporting," wrote Tait, who said he didn't view the document as "merely a name-dropping exercise."
"This document was about establishing a company to conduct opposition research on behalf of the campaign, but operating at a distance so as to avoid campaign reporting," Tait wrote.
"Indeed, the document says as much in black and white ... the combination of Smith's deep knowledge of the inner workings of the campaign, this document naming him in the 'Trump campaign' group, and the multiple references to needing to avoid campaign reporting suggested to me that the group was formed with the blessing of the Trump campaign," he said.
Smith committed suicide in May after talking to The Wall Street Journal about his experience.
Flynn was forced to resign as national security adviser in February after reports emerged that he had misled Vice President Mike Pence about his communication with Russia's former ambassador to the US, Sergei Kislyak. That communication is now under scrutiny by Mueller, who is also investigating Flynn's son, Michael Flynn Jr., according to NBC.