This story first published October 30 ahead of Tim Morrison's closed-door deposition. It has been updated with additional developments after his initial testimony.

Washington (CNN) Editor's note: This story first published October 30 ahead of Tim Morrison's closed-door deposition. It has been updated with additional developments after his initial testimony.

Tim Morrison, the former top Russia and Europe adviser on President Donald Trump's National Security Council, will testify publicly in the House Democrat-led impeachment inquiry on Tuesday.

Morrison has already corroborated key elements of the account of Bill Taylor , the top US diplomat in Ukraine, that Trump pressed for Ukraine to publicly announce investigations into former Vice President Joe Biden and his son, Hunter, using military aid the country sought to fight back against Russian aggression as leverage. There is no evidence of wrongdoing by either Biden.

He also told lawmakers that Gordon Sondland, the American envoy to the European Union, was acting at President Donald Trump's instruction in his dealings with Ukraine, and Sondland said that the President told him Ukrainian President Volodymyr Zelensky "must announce the opening of the investigations," according to a transcript of his deposition released Saturday.

Morrison's testimony has only fueled questions about Sondland's closed-door interview, which has been called into question based on the testimony others have given about his conversations with Trump. Sondland is scheduled to testify publicly before the House Intelligence Committee on Wednesday.

Morrison was the second White House official to testify who was on the July 25 phone call when Trump pressed Zelensky, to investigate the Bidens, according to a rough transcript of the conversation released by the White House and witness testimony of officials familiar with the situation.

It was one of Morrison's deputies, Alexander Vindman, who was the first official on the call to testify last month, telling lawmakers he raised concerns about the call to White House lawyers.

Morrison, a lawyer himself, joined the administration last July as the senior director of Weapons of Mass Destruction and Biodefense where he was intimately involved in the Russia and North Korea portfolios.

This summer, he was tapped by Bolton to replace Fiona Hill, who had been the White House's top official on Russian affairs. Hill testified before the committees earlier this month.

Creature of process

On the eve of his close-door testimony earlier this month, Morrison told his colleagues of his plans to leave the administration, a decision that was his and has been "planned for some time" given that he was an ally of former national security adviser John Bolton , who was fired by Trump in September, a source familiar said.

Morrison's hawkish views align with those of Bolton and he has been described as a creature of process by some close to him.

Bolton always told those who worked for him that process was their protector and sometimes you have to listen to the person elected -- advice Morrison adopted, sources said.

Morrison is a lifelong Republican described as a Reaganite and is referred to as "'Bolton's Bolton,' he is really hard right," according to one source familiar with Morrison.

A Baltimore native, Morrison attended law school at George Washington University and was planning to head to the Department of Justice when he graduated until an offer came in from former Arizona Republican Sen. Jon Kyl's office.

Morrison was impressed by Kyl's record so accepted the offer. He spent more than 17 years on the hill -- working for Kyl and the House Armed Services Committee -- before joining the NSC.

Bolton brought Morrison on at the NSC as a political appointee. The two men met over a decade ago when Morrison was working for Kyl and Bolton was Ambassador to the United Nations.

When Bolton was fired, Morrison kept his job. The two old allies have been in touch on a personal basis but it is unclear if they have discussed the Ukraine probe specifically, according to a source close to Morrison.

'It could have gone better'

After the July 25 call took place, Morrison informed Taylor , the top US diplomat in Ukraine, that it "could have gone better." He told Taylor that Trump suggested Zelensky and his staff meet with Trump's personal lawyer Rudy Giuliani and Attorney General William Barr, according to Taylor's testimony.

Taylor meticulously documented how he believed the White House had conditioned releasing security aid to Ukraine and providing a one-on-one meeting with Zelensky on Kiev publicly announcing an investigation that could help the President politically. He said that Sondland had told him that "everything" depended on the investigation being announced.

His opening statement mentioned Morrison 15 times by name.

Vindman , one of Morrison's deputies at the NSC, told lawmakers that he raised concerns about that phone call -- which lies at the heart of the Democrats' impeachment inquiry -- with lawyers at the White House. CNN has reported that multiple NSC officials could have raised concerns on the discussion, and Morrison could be one of them.

Morrison is also an integral player because he was serving in the top NSC role when security assistance to Ukraine was put on hold in mid-July due to a review announced internally by the Office of Management and Budget -- a review that CNN has reported never actually happened.

Morrison discussed Sondland's role

During his closed door interview, Morrison previously described how Sondland was a "problem" as he operated in what previous witness testimony described as irregular foreign policy channels. And he recalled Sondland speaking directly to Trump about the investigations and the military aid.

"He related to me he was acting -- he was discussing these matters with the President," Morrison said.

He said Sondland was a concern for his predecessor, former White House Russia expert Fiona Hill, who is also scheduled to testify this week.

"She described Ambassador Sondland as a problem," Morrison said, recounting a conversation they had about Sondland. "We both discussed that Ukraine was not in the EU, which led to the follow-on question of, why is he involved in Ukraine? And, as I mentioned, she mentioned Burisma, which I nearly did not know what that was."

Morrison explained their concern: "It was less about his role in Ukraine and more about how he conducted himself. He did not participate in the process. So we are very process-oriented on the NSC; we have a way we do things that works. And so when people come in and get involved in issues and they're not of that process, it creates risk."

After a Sept. 1 meeting between Zelensky and Vice President Mike Pence in Warsaw, Morrison testified he saw Sondland from across the room speaking with a top aide to Zelensky. Afterward, Sondland walked over to Morrison to tell him what he'd said.

"He told me that in his -- that what he communicated was that he believed the -- what could help them move the aid was if the prosecutor general would to go the mic and announce that he was opening the Burisma investigation," Morrison said, referring to the Ukrainian energy company that hired Joe Biden's son Hunter Biden.

Democrats seized on the information Taylor laid out as showing that there was a quid pro quo, but Taylor would not explicitly say that himself during his closed-door testimony, according to multiple sources familiar with what he said.

Taylor said that was a legal definition lawmakers should decide on, and he was just there to provide the facts.