The former national security adviser Michael Flynn. Thomson Reuters A cybersecurity researcher who described being recruited to vet hacked Hillary Clinton emails last year by a GOP operative tied to President Donald Trump's campaign team has been interviewed by the FBI's special counsel, Robert Mueller, Business Insider has learned.

Mueller interviewed Matt Tait, a former information-security specialist at Britain's Government Communications Headquarters who tweets as @pwnallthethings, several weeks ago, said 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, Peter Smith, and the former national security adviser Michael Flynn and whether Flynn played any role in seeking out the stolen emails during the election. Smith killed himself in May after talking to The Wall Street Journal about his experience.

The House Intelligence Committee has also interviewed Tait in its investigation into Russia's interference in the 2016 presidential election, CNN reported.

Tait declined to comment. A spokesman for Mueller's office also 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, adding that Smith 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 were pranksters or part of a larger Russian intelligence operation.

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 campaign and he seemed to know both Lt. Gen. Flynn and his son well."

Tait said Smith also listed the top Trump advisers Kellyanne Conway, Steve Bannon, and Sam Clovis as part of a group he had formed to conduct opposition research on Clinton, the 2016 Democratic presidential nominee.

Tait's interaction with Smith went on for at least a few weeks, Tait wrote, and at one point Smith sent him 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," Tait wrote, adding that 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," he continued. "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."

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 ambassador to the US at the time, Sergey Kislyak. That communication is now under scrutiny by Mueller, who is also investigating Flynn's son, Michael Flynn Jr., NBC reported last month.