(CNN) Top White House Ukraine exper t Alexander Vindman told congressional investigators he was convinced President Donald Trump was personally blocking $400 million in security aid to Ukraine to force that country to publicly announce an investigation into Joe Biden and his family, two sources present at the deposition told CNN.

Vindman, a decorated Army officer, on Tuesday testified that he was convinced that a quid pro quo existed by July 10, which was before Trump and Ukrainian President Volodymyr Zelensky in a phone call that is now at the heart of the House's impeachment inquiry. Trump has repeatedly said he did nothing improper on the call and has cited it as the sole reason for the impeachment inquiry.

Vindman believed the existence of a quid pro quo was clear during a July 10 meeting between American and Ukrainian officials. In his opening statement, Vindman wrote that date is when US Ambassador to the European Union Gordon Sondland told Ukrainian government officials that they would need to deliver "specific investigations in order to secure the meeting" with Trump that they so desired.

In a separate meeting of US officials immediately afterward, "Sondland emphasized the importance that Ukraine deliver the investigations into the 2016 election, the Bidens, and Burisma," Vindman testified.

But the fact that the $400 million in aid, including desperately needed military assistance, was also being used by the President didn't become clear until the next month, Vindman testified.

Then-national security adviser John Bolton instructed Vindman to prepare a decision memo by August 15 for Bolton and others in the administration to present a government-wide interagency argument to the President that he should release the assistance funds to Ukraine as soon as possible.

On August 16, Bolton and other senior Cabinet officials, including Secretary of State Mike Pompeo and Secretary of Defense Mark Esper, traveled to Trump's resort in Bedminster, New Jersey, to discuss a number of national security issues with the President.

Vindman learned after their meeting that Trump still refused to allow the security assistance funds to go to Ukraine, which made Vindman think the President was still waiting for the "deliverable," as Bill Taylor, the top American diplomat in Ukraine, referred to it in his own deposition -- the announcement of an investigation into the Bidens that Ukrainian officials needed to deliver in order to get the money they wanted.

While this conclusion by Vindman was perhaps the most consequential news to come out of his deposition, one of the sources present tells CNN, some House Republicans seemed more focused on trying to undermine the National Security Council member.

One of the sources present at the deposition describes Rep. Lee Zeldin, R-New York, as asking questions that seemed to suggest that by meeting with Ukrainian government officials, Vindman -- the point person on the White House National Security Council for Ukraine policy -- was perhaps involved in nefarious deeds. Vindman responded that it was his job to meet with those officials as a representative of the US government.

Zeldin also questioned why Vindman had not carried out the President's order to push for the Ukrainians to investigate the Bidens. Vindman said he felt the order was improper, according to that source present at the deposition. Vindman added that he would disregard an improper order from a general, too.

When Zeldin suggested that Vindman felt that way because carrying out the order would not be to the benefit of Ukraine, Vindman said he was not worried about how the move would affect Ukraine, but instead that he felt the order was wrong. He added that it also would not be to the benefit of Ukraine since such a move would undermine the bipartisan nature of American support for that country, but that a strong Ukraine was in America's national security interest.

"That's 100% untrue, and that's why you should be able to watch these depositions live," Zeldin told CNN when asked about the source's description. "That's why you should be able to read the transcrtipts immediately after."

Zeldin described multiple post-deposition leaks as "not true or they tell 3% of the story, or 30% of the story." Zeldin, a veteran, said that he could not comment on what he did ask about.

"Maybe it's my military training, maybe it's just the way the rules work," he said, "... but they say you cannot talk about the substance of the depositions. I would love to be able to give you additional insight. I wish I could tell you every single question that has was asked and every single answer that was given."

The same source present at the deposition said that Rep. John Ratcliffe, R-Texas, focused on the word "demand" in Vindman's response to Trump bringing up investigating the Bidens in his phone call with Zelensky.

"I did not think it was proper to demand that a foreign government investigate a U.S. citizen, and I was worried about the implications for the U.S. government's support of Ukraine," Vindman wrote in his opening statement.

Ratcliffe asked why Vindman believed it was a demand.

Vindman replied that when the most powerful person in the world asks the President of a country on the brink, such as Ukraine, to do something, he's going to do it.

Rachel Stephens, a spokeswoman for Ratcliffe, responded to the description by saying, "Chairman Schiff's rules prevent discussion of the actual testimony. I can clarify, however, that the source summary you provided is not accurate in several respects." She did not elaborate.

Two sources present at the deposition tell CNN that Zeldin, Ratcliffe and others asked a number of questions that seemed designed to figure out the identity of the whistleblower.