Former national security adviser John Bolton has retained legal counsel as Democrats on Capitol Hill signal interest in having him testify as part of their Trump impeachment inquiry, a source familiar with the situation told Fox News.

Bolton’s lawyers have had “a conversation“ with the House committees conducting the impeachment inquiry, but it has not progressed to the point of talking about an interview or deposition, the source said.

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The New York Times this week first reported the possibility of Bolton appearing before Congress for testimony, amid questions over what Bolton knew about President Trump's requests that Ukraine conduct investigations related to the Biden family and other matters and whether the requests were linked to military aid.

Bolton departed the White House last month over disagreements with the president on a range of foreign policy issues, including over a planned troop drawdown in Afghanistan.

Earlier this week, Bill Taylor, the top American diplomat to Ukraine, testified that he was told Bolton was concerned while serving in the White House about Trump’s actions toward Ukraine.

Taylor said he was told Bolton referred to this as a “drug deal."

Earlier this month, Fiona Hill, the former senior director for Russian and Eurasian affairs, told lawmakers that a July meeting with Ukrainian and U.S. officials about desired investigations left her and Bolton so concerned that he directed her to alert a lawyer in the National Security Council.

The interest in Bolton testifying comes as the three House committees leading the impeachment investigation are scheduled to hear from current and former staff of the National Security Council next week.

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Closed-door interviews are tentatively scheduled with Charles Kupperman, who served as a deputy to national security adviser Bolton, and Tim Morrison, the NSC's current Russia and Europe director. Kupperman, like Bolton, has left the White House.

The Associated Press contributed to this report.