U.S. Special Counsel Robert Mueller will accept written answers from President Donald Trump on whether his campaign conspired with Russia to interfere in the 2016 U.S. election, but Mueller is not ruling out a follow-up interview on that issue, a person familiar with the matter said on Tuesday.

Mueller’s offer to accept written responses from Trump on questions about possible collusion was contained in a letter that Trump’s lawyers received on Friday, Reuters reports citing its source.

The letter was first reported by the New York Times.

Trump’s legal team and Mueller’s investigators have been negotiating for months over whether Trump will be formally interviewed in the probe.

After receiving the written responses, Mueller’s investigators would decide on a next step, which could include an interview with Trump, the person said.

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Another area of interest to Mueller’s office is whether Trump tried to obstruct the investigation, but it was not clear how any possible questions on that subject might be handled.

A spokesman for Mueller’s office declined to comment.

Rudy Giuliani, one of Trump’s lawyers, did not immediately respond to a request for comment.

U.S. intelligence agencies have concluded that Russia interfered in the 2016 presidential race, seeking to tilt the vote in Trump’s favor.

Russia has denied interfering in the election. Trump has said there was no collusion and has called the Mueller probe a “witch hunt.”