The whistleblower's account of the effort by President Donald Trump, the president's personal lawyer, Rudy Giuliani, and administration officials to push Ukraine to pursue investigations that would benefit Trump politically became a road map for the House's impeachment investigation and has been largely corroborated by subsequent testimony.
The FBI is interested in the "substance" of the whistleblower's complaint, said one person familiar with the matter.
But no interview has yet to be scheduled.
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The development, first reported by Yahoo News, adds a wrinkle to the political debate over whether Trump's conduct constitutes an impeachable offense - a question at the center a fiercely partisan fight that the Federal Bureau of Investigation has so far avoided.
The FBI did not immediately respond to a request for comment.
Trump and his allies have repeatedly demanded that they be told the identity of the whistleblower and be able to confront him or her. During the call, Trump pressed for investigations related to Democratic rival Joe Biden, his son Hunter, and their business dealings in that country, among other things.
The person said it was clear from the FBI that the whistleblower was not regarded as the target of any investigation but rather a potential witness.
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The FBI has also refused to comment on the matter.
The lawyers have offered to make their client available to answer congressional investigators' written questions under oath, but lawmakers have yet to accept that offer.
Republicans on the committee conducting the impeachment inquiry have attempted to force the whistleblower to come forward and testify in public hearings, a move that lawyers for the agent and Democrats in Congress have so far rejected.
"Please stop", he said when Nunes protested again. If the witness has a good faith belief that this may reveal the identity of the whistleblower, that is not the objective that we're here for.
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According to Sondland, Vice President Mike Pence and Secretary of State Mike Pompeo were aware of the Ukraine pressure campaign. Sondland said that was indeed the case and that the president only cares about the "big stuff". "Everyone was in the loop".