Special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's office is reportedly probing the Trump campaign's relationship with the National Rifle Association (NRA) in the run-up to the 2016 election.

Former Trump campaign aide Sam Nunberg told CNN on Tuesday that Mueller's team of investigators was looking into the circumstances that led to Donald Trump speaking at the group's annual convention in 2015 prior to announcing his bid for the presidency.

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"When I was interviewed by the special counsel's office, I was asked about the Trump campaign and our dealings with the NRA," Nunberg told the outlet.

A spokesperson for the special counsel's office declined to comment to CNN. The NRA did not immediately return a request for comment from The Hill.

CNN reported Tuesday that Mueller's team was probing the organization as early as February 2018, when Nunberg sat for an interview with the special counsel's office.

President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE was not pressed on a possible connection with the NRA in written questions sent to him by Mueller's team, a source told CNN.

Trump spoke at the group's national convention in Tennessee in 2015, clinching the gun rights group's endorsement in May 2016. The group spent more than $30 million to support his candidacy, the outlet noted.

The NRA has come under scrutiny for its connections to Russian nationals.

Questions about the group's potential contacts with Russia were raised last year after Maria Butina, a Russian woman and gun rights activist arrested and charged with acting as an unregistered agent of the Russian government in the U.S., pleaded guilty last month to engaging in conspiracy against the U.S.

Butina admitted in the District Court for the District of Columbia that she and an American, known in court documents as "U.S. Person 1," conspired with and acted under the direction of a Russian government official to establish unofficial lines of communications with people able to influence U.S. politics leading up to the 2016 presidential election.

Butina is a former assistant to Alexander Torshin, a deputy governor of the Central Bank of Russia, that Senate Judiciary Committee Democrats allege potentially funneled Russian money into Trump’s 2016 campaign through the NRA.

Updated at 5:02 p.m.