Washington (CNN) Prospects are growing increasingly grim that congressional investigators will have the opportunity to interview the anonymous whistleblower whose complaint sparked an impeachment inquiry into President Donald Trump over allegations he abused his office in his dealings with Ukraine.

The attorneys for the whistleblower who lodged a complaint with the intelligence community inspector general related to Trump's July 25 phone call with Ukraine's President wrote in a Washington Post op-ed Friday that there is no need for their client to testify in person, even behind closed doors.

Andrew Bakaj and Mark Zaid argue that their client's anonymity needs to be protected and say he has no additional information to offer about the call, writing: "Because our client has no additional information about the president's call, there is no justification for exposing their identity and all the risks that would follow."

On Thursday, the lawyers had released an October 8 letter to leaders of the Senate Intelligence Committee in which they say an in-person meeting with committee staff is a "non-starter," citing the need to protect the whistleblower's safety.

"Additionally, it is our firm position that we must treat Congress as a whole in a fair and impartial manner. Thus, given the events of just the past 24 hours, let alone the past three weeks -- to include threats from the President of the United States -- our client is now only willing to communicate with both Committees through written interrogatories," the letter states.

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