Just a day before he was scheduled to testify in the ongoing House impeachment inquiry, a top Russia expert at the National Security Council has resigned from the White House, according to multiple news outlets Wednesday including NPR.

Tim Morrison played a key role in the events described by Ambassador Bill Taylor's impeachment testimony, and the NSC aide is expected to corroborate at least some of Taylor's account.

Morrison was brought into his role by former National Security Adviser John Bolton, who left the White House under contentious circumstances in September before the Ukraine scandal burst open. Morrison replaced Fiona Hill, who has also testified in the impeachment proceedings.

"Had heard this was coming and was part of an ongoing effort to root out Bolton loyalists," said Tom Wright of the Brookings Institution on Twitter.

NPR explained Morrison's role in the events at the center of the impeachment inquiry:

Morrison had alerted NSC lawyers about alleged demands being placed on the Ukrainian government to investigate a company where the son of former Vice President Joe Biden sat on the board, according to testimony this month from William Taylor, the top U.S. diplomat in Ukraine.

Taylor testified that Morrison told him that Gordon Sondland, Trump's ambassador to the European Union, had informed a top Ukrainian official that U.S. military aid hinged on Zelenskiy's committing to investigating the firm connected with Biden's son.

NPR did not confirm whether he left of his own volition or was forced out. But he's not the first staffer wrapped up the Ukraine drama to find himself out of a job. As the State Department's role in the president's dealings with Ukraine were coming to light, U.S. envoy Kurt Volker suddenly departed the administration last month.