Over a jam-packed, nearly 12-hour stretch on Tuesday, four key figures at the center of the Democratic-led impeachment inquiry testified publicly before the House Intelligence Committee.

First, Lt. Col. Alexander Vindman, the top Ukraine expert on the National Security Council, and Jennifer Williams, a special adviser on Europe and Russia to Vice President Mike Pence — who both listened in on the July 25 call between President Donald Trump and Ukrainian President Volodomyr Zelenskiy — said that call gave them cause for concern, while Vindman faced repeated personal attacks by Republicans on the committee.

Next, Kurt Volker, the former U.S. special envoy to Ukraine who resigned after his name appeared in the whistleblower complaint about Trump’s dealings with Ukraine, made a significant revision to his testimony, and Tim Morrison, a former National Security Council staffer, expressed worry about ties between military aid to Ukraine and the opening of investigations that would be politically advantageous to Trump.

Here are our 10 takeaways from today's public hearings.

1. Volker changes testimony about alleged quid pro quo

Volker amended his testimony from his Oct. 3 closed-door deposition, telling the members of the committee Tuesday that he now sees that others in the Trump administration sought an investigation into the Biden family and that they told Ukraine's government that millions in military aid depended on it.

Volker was the first person to talk to impeachment investigators behind closed doors, on Oct. 3, and he said this shift reflected "a great deal of additional information and perspectives" that has come to light.

"I have learned many things that I did not know at the time of the events in question," Volker said in his opening statement. "I did not know of any linkage between the hold on security assistance and Ukraine pursuing investigations. No one had ever said that to me — and I never conveyed such a linkage to the Ukrainians."

Volker said in his Oct. 3 deposition that "official representatives" of the U.S. had “never communicated to Ukrainians" that the aid had been suspended for a specific reason. He added, "We never had a reason."

But in the weeks since, multiple other officials testified that the ambassador to the European Union, Gordon Sondland, had, in fact, told a top Ukrainian presidential aide that the country would not likely get the money unless the investigations were announced, a fact Sondland himself ultimately conceded.

This was a significant shift by Volker and it reinforces the emerging narrative being established by House Democrats that military aid for Ukraine was, in fact, conditioned on the launching of specific investigations by Ukraine.

2. Volker had an 'aha!' moment on Biden-Burisma connection

Volker also said in his public testimony Tuesday that, in hindsight, he now understands the investigation desired by the White House into Burisma Holdings, the Ukrainian gas company with a history of shady dealings, was, in fact, intended as an investigation into the Bidens.

While he echoed his previous testimony that "at no time" did he know of or participate in an effort "to urge Ukraine to investigate" Biden, he also backed away from his previous insistence that he had no reason to think that the interest of Trump and his personal lawyer Rudy Giuliani in Burisma had to do with Biden, a potential 2020 opponent of the president.

"In hindsight, I now understand that others saw the idea of investigating possible corruption involving the Ukrainian company Burisma as equivalent to investigating former Vice President Biden. I saw them as very different — the former being appropriate and unremarkable, the latter being unacceptable," Volker said, adding that "in retrospect, I should have seen that connection differently, and had I done so, I would have raised my own objections" to a Trump administration push to investigate the Bidens.

In amending his testimony, Volker joined Bill Taylor, the top U.S. diplomat in Ukraine, and Sondland to become the third impeachment inquiry witness to provide additional information hearing beyond what they testified to in their closed-door depositions.

3. July 25 call was 'unusual,' 'improper'

Vindman and Williams offered stark, critical assessments of the July 25 call between Trump and Zelenskiy, which they listened in on as part of their normal duties.

Vindman said he was "concerned" with what he'd heard on the call and that he felt it was "improper for the president of the United States to demand a foreign government investigate a U.S. citizen and political opponent."

Williams said she "found the July 25th phone call unusual because, in contrast to other presidential calls I had observed, it involved discussion of what appeared to be a domestic political matter."

Both Vindman and Williams, the first two witnesses who heard the call firsthand to testify publicly, had made similar criticisms during their closed-door testimony last month. But doing so again publicly, and with such unambiguous language, set the tone early in Tuesday’s hearing for Democrats trying to paint a picture that what occurred was harmful to the country.

4. Robust efforts to unmask — and protect — the whistleblower

House Intelligence Republicans repeatedly used their time to question Vindman about who he told about his concerns surrounding the July 25 call — an apparent effort to get him to unmask the still-unnamed intelligence community whistleblower.

During two particular exchanges, Rep. Jim Jordan, R-Ohio, and ranking member Devin Nunes, R-Calif., suggested Vindman had contact with the whistleblower, despite having said in his closed-door testimony that he did not know the identity of the whistleblower.

Both times on Tuesday, Vindman said he'd simply spoken with someone in the intelligence community. His lawyer also interjected to advise his client not to answer.

Schiff also cut in on both occasions to instruct everyone present that the “committee will not be used to out the whistleblower.”

Republicans have claimed — falsely — that Democrats coordinated with the whistleblower ahead of his or her filing the formal whistleblower complaint and have demanded the whistleblower testify.

5. Vindman gets personal

A poignant, poised and at times visibly nervous Vindman — who was born in Kyiv, then part of the USSR, and fled with his family to the U.S. as a child — used a large part of his opening statement to deliver a stunningly personal message about how his family had come to America for a better life and how escaping an authoritarian regime instilled in him and his brothers a sense of duty to serve in the U.S. military.

He said that he never expected to testify about the president’s actions but he did so out of a "sense of duty" and said he recognized that his actions "would not be tolerated in many places around the world."

“In Russia, my act of expressing my concerns to the chain of command in an official and private channel would have severe personal and professional repercussions and offering public testimony involving the president would surely cost me my life,” he said. (Trump, in fact, has already publicly criticized at least two of the witnesses — Williams and Marie Yovanovitch — in the impeachment inquiry and said earlier Tuesday he didn't know Vindman.)

Addressing his father, Vindman concluded his statement by saying that “you made the right decision 40 years ago to leave the Soviet Union and come here to the United States of America in search of a better life for our family.”

“Do not worry, I will be fine for telling the truth,” Vindman said.

6. Republicans question Vindman’s loyalty

Nevertheless, Vindman, an Army lieutenant colonel who received a Purple Heart after he was wounded by an improvised explosive device in Iraq in 2004, faced repeated character attacks from several House Intelligence Republicans.

In one case, Steve Castor, the counsel for committee Republicans, asked a series of questions about whether Vindman had at one point been offered the post of Ukrainian defense minister by a Ukrainian politician.

Vindman, for his part, said such a request occurred three times, but that he dismissed the offers immediately, reported them to his superiors and to counterintelligence authorities, and told Castor it's no secret where his allegiance is.

"I'm an American," he said.

That the topic came up at all seemed to be part of a clear effort by Republican to discredit the allegiance of Vindman. Several conservatives have used the same tactic, including Fox News personalities.

At another point, Jordan asked Vindman about comments from Tim Morrison, another National Security Council official, who expressed skepticism about Vindman’s judgment.

Vindman, in this case, was prepared for the attack and responded by reading from a recent performance review filed by Trump’s former top Russia analyst Fiona Hill, which praised his abilities and labeled him a top military official.

7. First testimony about secret server

The September release of the whistleblower complaint revealed the existence of a secret electronic filing system that White House officials used, perhaps improperly, to “lock down” the transcript of the July 25 call.The whistleblower claimed in the complaint that the server was designed to house sensitive national security information, not politically sensitive information, and that its use for the latter constituted an “abuse.”

In what amounted to the first account in public testimony by witnesses in the impeachment inquiry, Vindman explained under questioning that the summary of the July 25 call was transferred to a private, more secure server “to avoid leaks” and to help “preserve the integrity of the transcript.”

Vindman, however, said he “didn’t take it as anything nefarious” and that “concerns about leaks … seemed valid,” but that the decision to have it “segregated into a separate security system” was “made on the fly.”

Meanwhile, Vindman, at another point, also testified that Burisma, the Ukrainian gas company that Hunter Biden joined as a board member in 2014, had come up during the July 25 call — even though the summary of the call released by the White House did not. Vindman said he’d recommended that the summary mention Burisma, but told the committee that there was nothing “nefarious” about that mention being omitted.

8. 'Administrative error,' Morrison says of the server

After being asked by the attorney for the Democratic members about the highly unusual effort to “lock down” the summary of the July 25 call between Trump and Zelenskiy, Morrison responded that it was an "administrative error."

In his closed-door testimony, the transcript of which was released Saturday night, Morrison said that he was told by John Eisenberg, the National Security Council's legal counsel, that doing so “was a mistake.”

That testimony would appear to contradict allegations by the whistleblower.

9. Trump's concerns about corruption

Vindman testified that he’d prepared talking points for Trump for his April phone call with Zelenskiy on which Trump congratulated his counterpart on his election win.

Those talking points, Vindman testified Tuesday, included addressing corruption in Ukraine.

Trump, however, did not address corruption on the call, according to a record of the call released by the White House last week — even though a readout of the call released this year by the White House stated that Trump had expressed his commitment to working with Ukraine “to implement reforms that strengthen democracy, increase prosperity and root out corruption.”

Vindman said Tuesday that he wouldn’t call the readout false, but rather "not entirely accurate," because readouts are often used as messaging to promote policies consistent with U.S. policy and indicate what is important to an administration.

Nevertheless, that Trump didn’t bring up corruption, despite the wishes of his advisers, contradicts the White House’s claims that Trump’s desire to see an investigation into Burisma was merely part and parcel of a broader concern within the administration over widespread corruption in the country.

10. A bad day for Trump, Republicans

As NBC News' Jon Allen points out in an analysis, Democratic staff lawyer Daniel Goldman, with calm and persistent questioning, drove a wedge between the witnesses and Trump.

Goldman's questioning during the second hearing led Morrison to describe a "parallel" policy process in which Sondland spoke directly with Trump — who has said that he barely knows Sondland — and that he repeatedly checked up on Sondland’s claims that he had spoken with Trump and found those claims to be true.

"I was concerned" about Sondland connecting aid to investigations, Morrison said.

That, along with Volker’s testimony, deeply damages the narrative put forth by some Republicans that Sondland and Giuliani were operating outside the president's knowledge and to the case, put forth by the administration and many Republicans, that the aid was not withheld as leverage to produce a public statement regarding investigations.

Bonus: Volker says he's not an amigo

Volker also said he was "never aware” of “any designation by President Trump or anyone else” putting himself, Sondland or Energy Secretary Rick Perry “or the three of us as a group in charge of Ukraine policy.”

Taylor and other witnesses had previously testified that Giuliani, a private citizen, was involved in a shadow diplomacy team that included Sondland, Volker and Perry. George Kent, a deputy assistant secretary of state who worked on Ukraine and five other countries, previously testified that Volker, Sondland and Perry called themselves "the three amigos.”

Volker’s denial puts his testimony, once again, at odds with that of other crucial figures.