The Trump administration has been clear: Officials are not to comply with Congress' demands for documents and witness testimony in its impeachment inquiry.

But it looks like the people around President Donald Trump are done taking his orders, and the dam is bursting as a flurry of current and former officials step up to testify.

Taken together, their revelations paint a damaging portrait of a concerted effort across the government to leverage US foreign policy in exchange for material that would personally benefit the president.

More whistleblowers are also coming out of the shadows. In addition to three who have already been reported on, Congress has heard from several more in recent days, some of whom were determined to speak out after learning of Trump's efforts to use official policy for political gain.

Visit Business Insider's homepage for more stories.

The Trump administration was unequivocal: Government officials are not to comply with Congress' demands for documents and witness testimony as they investigate whether to impeach President Donald Trump.

But it looks like the people around Trump are done taking his orders, and the dam is finally bursting.

It started in August, when a US intelligence official filed an unprecedented whistleblower complaint accusing the president of using his public office for private gain.

Specifically, the official alleged that Trump repeatedly pressured Ukrainian President Volodymyr Zelensky during a July 25 phone call to investigate former Vice President Joe Biden and his son Hunter for corruption. Trump also asked Zelensky to help discredit the FBI's finding that Russia secretly worked to help elect Trump.

Beyond asking a foreign power for dirt against a political rival ahead of an election, Trump is also battling allegations that he held up a nearly $400 million military-aid package to Ukraine days before the phone call to maintain leverage over Zelensky.

In the weeks since, Trump officials have come out of the woodwork, in defiance of his and other top officials' orders to stay silent, to offer their own testimony to lawmakers in the impeachment inquiry.

Taken together, their revelations paint a damaging portrait of a concerted effort across the Trump administration to leverage US foreign policy in exchange for material that would personally benefit the president.

Read more: These are the key players you need to know to make sense of the Trump impeachment inquiry

Kurt Volker. Pavlo Gonchar/SOPA Images/LightRocket via Getty Images

Ex-diplomat reveals the extent to which senior US officials were involved in Giuliani's efforts

The first official to testify was Kurt Volker, the US's former special representative to Ukraine. The most significant part of Volker's hearing was not his testimony but a series of explosive text messages he turned over to Congress.

The messages — exchanged between Volker; US Ambassador to the EU Gordon Sondland; Bill Taylor, the US's chief diplomat in Ukraine; and the president's personal lawyer Rudy Giuliani — revealed how intricately senior US officials were involved in Giuliani's efforts to get dirt on the Bidens.

According to the texts, Volker was instrumental in establishing a channel of communication between Giuliani and Andriy Yermak, a key aide to Zelensky.

Volker and Sondland also exchanged several texts in which they both expressed the understanding that a good relationship between Trump and Zelensky was predicated on the Ukrainian government pursuing "investigations" and "getting to the bottom" of what happened in 2016. The two men went as far as to put together a draft statement for Zelensky to release committing to the investigations Trump wanted as a precondition to a White House meeting.

The messages directly contradicted the president's claims that there was no quid pro quo involved in his interactions with Zelensky.

Read more: Prosecutors are scouring Rudy Giuliani's bank records and business dealings in Ukraine as part of a widening criminal investigation

Trump's main defender prepares to throw a wrench into his claims

Gordon Sondland. Photo by DANIEL MIHAILESCU/AFP/Getty Images

Sondland, meanwhile, has emerged as a key player, based not only on his role in defending Trump in the text messages but also on his conversations with the president.

The ambassador will testify before House lawmakers on Thursday after initially being blocked from doing so by White House lawyers.

The most noteworthy texts Sondland exchanged were with Taylor, a career diplomat who raised red flags about the way the Trump administration was handling the US's relationship with Ukraine and its newly elected government.

The most stark message from Taylor came on August 30, after it became clear that the administration was withholding military aid while talks between Trump and Zelensky were ongoing.

"Are we now saying that security assistance and WH meeting are conditioned on investigations?" Taylor texted Sondland.

"Call me," Sondland replied.

Read more: 2 of Rudy Giuliani's associates who prosecutors say helped him dig up dirt on Joe Biden have been charged with campaign finance violations

Over the next several days, Taylor continued sounding the alarm and even threatened to quit if Zelensky did as Trump asked and the US president still withheld the military aid.

"As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign," Taylor texted Sondland on September 9.

Five hours later, Sondland came back with a lengthy response.

"I Believe you are incorrect about President Trump's intentions," he wrote. "The President has been clear no quid pro quo's of any kind. The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelensky promised during his campaign I suggest we stop the back and forth by text If you still have concerns I recommend you give Lisa Kenna or a call to discuss them directly. Thanks."

Trump and his allies point to Sondland's message saying there were "no quid pro quo's of any kind" as proof that nothing untoward happened during Trump's conversation with Zelensky.

But Sondland appears to be ready to cast doubt on Trump's claims this week.

Specifically, he's planning on testifying to lawmakers that the contents of his text to Taylor were relayed directly to him by Trump in a phone call, The Washington Post reported.

"It's only true that the president said it, not that it was the truth," Sondland is preparing to testify, according to The Post. He is also expected to contradict the president's claims denying a quid pro quo and acknowledge that Trump's efforts did, in fact, represent a quid pro quo "but not a corrupt one," according to The Post.

The Post reported that Sondland's testimony would also shed light on Trump's backing off from his efforts when the public learned of his withholding military aid and reluctance to meet with Zelensky one-on-one. Sondland additionally plans to tell lawmakers that his efforts to get Ukraine to publicly commit to investigating Burisma Holdings — the Ukrainian oil and gas company whose board Biden's son served on — were undertaken at Giuliani's direction.

And he is expected to testify that he didn't know Trump's push to investigate Burisma was really connected to the Bidens until the whistleblower complaint was released. Whether that claim will withstand scrutiny remains to be seen, given Sondland's direct access to Trump throughout his tenure as the US's ambassador to the EU.

Read more: A Fox News poll just found that more than half of American voters want Trump impeached and removed from office

Former US Ambassador to Ukraine Marie Yovanovitch. Scott J. Applewhite/AP

Ex-Ukraine envoy defies State Department to testify

Perhaps the most important witness to testify against Trump and Giuliani so far is Marie Yovanovitch, the US's former ambassador to Ukraine who was abruptly recalled this year.

Yovanovitch talked to lawmakers on Friday despite being ordered by the State Department not to cooperate with the impeachment inquiry. House Democrats learned Yovanovitch had been blocked from appearing and served her with a subpoena, after which she testified.

Based on what she told lawmakers, it's clear why the Trump administration didn't want Yovanovitch to talk.

Yovanovitch was recalled from Ukraine in May, while Giuliani was urging Ukrainian government officials to investigate baseless claims of corruption against Biden and his son.

Yovanovitch testified to Congress that she was removed from her position based on "unfounded and false claims by people with clearly questionable motives."

"I do not know Mr. Giuliani's motives for attacking me," she said. But people associated with Giuliani "may well have believed that their personal financial ambitions were stymied by our anti-corruption policy in Ukraine."

Yovanovitch was tough on Yuriy Lutsenko, Ukraine's former prosecutor general whom Giuliani favored, and the divide between her and Giuliani widened when she wouldn't help Giuliani look for dirt on Hunter Biden ahead of the 2020 election.

Read more: Trump's approach to the impeachment inquiry bears a striking resemblance to how the mafia operates

Yovanovitch also said she was told by a top State Department official that the president had pushed for her removal even though the State Department believed she had "done nothing wrong." According to The New York Times, Yovanovitch testified that John Sullivan, the deputy secretary of state, told her earlier this year that "this was not like other situations where he had recalled ambassadors for cause."

Instead, Sullivan reportedly told Yovanovitch that Trump had "lost confidence in me and no longer wished me to serve as his ambassador." She was also said to have testified that there'd been "a concerted campaign against me" and that the department "had been under pressure from the president to remove me since the summer of 2018."

Yovanovitch's ouster is central to a federal criminal investigation being conducted by the Southern District of New York. Among other things, prosecutors are looking into whether Giuliani was working on Lutsenko's behalf when he called for Yovanovitch's removal and violated foreign-lobbying laws in the process of doing so.

Former national security adviser John Bolton. Denis Balibouse/Reuters

A top Russia analyst defies the White House in testifying, arguing there's no executive privilege in cases of 'government misconduct'

On Monday, the White House's former top Russia analyst provided a window into how Giuliani and his associates' pressure campaign in Ukraine was received within the White House.

Fiona Hill served as the senior director for Russian and Eurasian affairs from April 2017 until August. She testified this week that John Bolton, the former US national security adviser, was so alarmed by efforts to pressure Ukraine for dirt that he instructed Hill to inform White House lawyers he was not part of the plan, according to The New York Times.

Bolton's directive came after he got into a heated July 10 exchange with Sondland, who was part of Giuliani's shadow foreign policy on Ukraine.

Read more: John Bolton described Rudy Giuliani as a 'hand grenade' that would 'blow everybody up'

"I am not part of whatever drug deal Sondland and Mulvaney are cooking up," Bolton told Hill to tell White House lawyers, according to Hill's testimony cited by The Times. And he directed Hill to inform the lawyers that Giuliani, Sondland, and Mulvaney were part of a rogue operation with serious legal implications, Hill testified, according to The Times.

She also said Bolton described Giuliani as a "hand grenade" that would "blow everybody up."

CNN reported further that Hill told Congress she witnessed "wrongdoing" while serving in the Trump administration and tried to bring it to the attention of White House lawyers.

Hill's testimony came after claims of executive privilege from the White House, and a letter that her attorneys sent to White House lawyers in response revealed even more about the nature of her testimony.

"The deliberative process privilege disappears altogether when there is any reason to believe government misconduct occurred," Hill's lawyers wrote. "This appears to be a foundational principle of our nation's constitutional system of governance."

More whistleblowers come out of the shadows

Rudy Giuliani. Associated Press

Lawmakers expect to hear from several other officials in the coming days. George Kent, the deputy assistant secretary of state for Eurasia who has a deep knowledge of Ukraine, appeared to testify on Tuesday. On Monday, House Democrats also scheduled an interview with Laura Cooper, a deputy assistant secretary of defense for Russia, Ukraine, and Eurasia policy.

Meanwhile, the president could face even more whistleblowers as other government officials consider stepping forward. The attorneys representing the first Ukraine whistleblower have already said they're representing a second US intelligence official who has firsthand knowledge of some of the events surrounding Trump's call with Zelensky.

And the House Ways and Means Committee is trying to learn more information about a third whistleblower, who works in the IRS and whose complaint alleges "inappropriate efforts to influence" the agency's audit of Trump's tax returns, according to a court filing from the committee. According to The Post, the person accused of trying to interfere with the audit is a political appointee at the Treasury Department.

The Daily Beast also reported, citing two congressional sources, that new potential whistleblowers are coming forward in the wake of the House's impeachment inquiry.

It's unclear what or how accurate these people's claims are, but the report said they were emboldened by the actions of the first whistleblower. They were also spurred to speak out after learning more about Trump's efforts to use official policy for personal gain, according to The Daily Beast.