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(CNN) —  

Tom Barrack, one of President Donald Trump’s close friends and former chairman of his inaugural committee, was interviewed by the special counsel’s team last December, a source familiar with the matter told CNN.

Barrack was asked largely about former Trump campaign chairman Paul Manafort and his deputy, Rick Gates, and about Barrack’s relationship with both of them, the source said. Manafort has pleaded not guilty to charges brought by special counsel Robert Mueller on money laundering, being an unregistered foreign agent and failing to file reports of foreign banks and financial accounts.

Gates has pleaded guilty to charges of financial fraud and lying to investigators.

Barrack introduced Manafort to Trump, and Barrack has known Trump for 40 years. The source said Barrack’s relationship with Gates began during the campaign. He hired Gates for Trump’s inauguration, and when that was over, the contract ended. The source said federal investigators did not ask him about money, including inaugural money, or his relationship with the President, and it was made clear he was not a target of the probe. The interview lasted about two hours, the source said.

The Associated Press first reported the interview.

Mueller’s federal investigation is probing Russian interference in the 2016 presidential election and any potential ties between Trump’s campaign associates and the Russian government.

The President has repeatedly referred to the investigation as a “witch hunt” and denied any collusion between his campaign and Russia.

Barrack, the executive chairman at real estate investment trust Colony NorthStar, is a business associate of Trump who has helped raise funds in support of Trump’s political ambitions and has publicly defended the President.

Barrack called Trump a “revolutionary” and “a warrior of sorts,” in a CNN interview last year.

At the 2016 Republican National Convention, he said that Trump is one of his “closest friends,” and described him as “good enough, tough enough, smart enough and well-versed enough” to win the presidency.