The three House committees leading an impeachment investigation are scheduled to hear from current and former staff of the National Security Council next week, according to four people familiar with the inquiry's schedule.
Closed-door interviews are tentatively scheduled with Charles Kupperman, a deputy to national security adviser John Bolton, and Tim Morrison, NSC's current Russia and Europe director. Kupperman and Bolton have both left the White House.
The four people spoke to The Associated Press on condition of anonymity to discuss the confidential schedule.
The Democrats are investigating President Donald Trump's requests that Ukraine conduct certain investigations and whether the requests were in exchange for military aid. Trump requested the probes on a July phone call with Ukrainian President Volodymyr Zelenskiy, and both Kupperman and Morrison may have been listening into the call because of their positions in the White House.
Kupperman's interview is scheduled for Monday, and Morrison's is set for Thursday. If Morrison appears for the interview, he will be the first White House aide to testify even as Trump has said his administration would not cooperate.
Morrison could be a key witness. Top Ukraine diplomat William Taylor told lawmakers in his deposition Tuesday about phone calls he had with Morrison that described the Ukraine effort.
In a detailed opening statement, Taylor said Morrison told him in an August phone call that the "president doesn't want to provide any assistance at all," speaking about the military aid that had already been authorized by Congress.
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"That was extremely troubling to me," Taylor said.
Taylor described another call with Morrison in September in which Morrison told him of a conversation another diplomat, Gordon Sondland, had held with a Ukrainian official. Morrison told Taylor that Sondland had told the Ukrainians that the aid money wouldn't come until Zelenskiy pursued an investigation into a company linked to former Vice President Joe Biden's family. Biden is running for the Democratic nomination to challenge Trump.
Those conditions would contradict Trump's own claims that there was no "quid pro quo" for the investigations.