Washington (CNN) White House acting chief of staff Mick Mulvaney made a stunning admission Thursday by confirming that President Donald Trump froze nearly $400 million in US security aid to Ukraine in part to pressure that country into investigating Democrats.

Hours later, Mulvaney then denied ever saying those words.

The dramatic admission came during an afternoon news conference where Mulvaney insisted that he knew only of a US request to investigate the handling of a Democratic National Committee server hacked in the 2016 election, but text messages between US diplomats show efforts to get Ukraine to commit to an investigation into Burisma, the company on whose board former Vice President Joe Biden's son sat. There is no evidence of wrongdoing in Ukraine by either Biden.

"That's why we held up the money," Mulvaney said after listing the 2016-related investigation and Trump's broader concerns about corruption in Ukraine.

After weeks during which Trump denied the existence of any political quid pro quo in his withholding of security aid to Ukraine, Mulvaney confirmed the existence of a quid pro quo and offered this retort: "Get over it."

"We do that all the time with foreign policy," Mulvaney said of the influence of politics in the Trump administration.

Then on Thursday evening, Mulvaney attempted to claim that he did not admit to the quid pro quo despite clearly being asked if the Trump administration withheld funding for Ukraine for an investigation into the DNC server and answering affirmatively.

"The only reasons we were holding the money was because of concern about lack of support from other nations and concerns over corruption," Mulvaney said in a written statement, adding, "There never was any condition on the flow of the aid related to the matter of the DNC server."

In an unusual statement expressing public distance from the White House, a senior Justice Department official responded: "If the White House was withholding aid in regards to the cooperation of any investigation at the Department of Justice, that is news to us."

Trump's attorney Jay Sekulow told CNN's Jim Acosta: "The legal team was not involved in the acting chief of staff's press briefing."

Testimony

Mulvaney's admission came after current and former Trump administration officials revealed in congressional testimony that a White House meeting between Trump and Ukrainian President Volodymyr Zelensky was also being conditioned on Ukraine launching politically charged investigations.

Mulvaney said he does not recall any "serious" conversations about setting up a White House meeting, but a senior White House official told CNN that Mulvaney was likely aware that the meeting was being conditioned on investigating matters related to the 2016 election, although not about the Bidens.

Efforts by Trump administration officials to pressure Ukraine into carrying out investigations into 2016 and the Bidens alarmed diplomatic and national security officials across the government.

For then-national security adviser John Bolton and Fiona Hill, Trump's top foreign adviser on Europe and Russia, July 10 brought the clearest sign yet that politics was troublingly being injected into US foreign policy toward Ukraine.

Meeting with Ukrainian officials that day, Gordon Sondland, the US ambassador to the European Union and a top Trump donor , made clear that the Ukrainian President could secure a White House visit if his government opened investigations -- including one into Burisma, the energy company on whose board Biden's son Hunter once sat -- that would be a political boon for Trump

The apparent quid pro quo so alarmed Bolton that he described the interaction at the time as "a drug deal" and urged Hill, who attended the meeting, to report their concerns to White House lawyers.

It would be just one of several instances where National Security Council and intelligence officials raised concerns with attorneys inside the Trump administration, believing they were witnessing a hijacking of Ukraine policy by the President's political allies for his political gain.

The ultimate inflection point -- which prompted an intelligence officials to file a whistleblower complaint that triggered the current House impeachment inquiry -- came during a phone call between Trump and Zelensky on July 25, in which Trump urged Zelensky to investigate 2016 election matters and Biden, the former vice president and the leading Democratic candidate for President at the time.

Concerns raised early

Much of the attention surrounding Democrats' current impeachment efforts has focused on that call and its aftermath. But the efforts by some officials to pressure Ukraine into investigating matters that would be politically beneficial to Trump and the concerns raised by other officials in the weeks before the call have become a focal point as lawmakers assess whether to impeach the President.

Mulvaney's role in the lead-up to that call is swelling.

Mulvaney was not only aware of the linkage between the withheld security aid to Ukraine and the political investigations Trump was seeking, but he was the official who carried out Trump's directive to freeze nearly $400 million in congressionally approved security aid to Ukraine.

He also led a meeting putting key officials -- including Sondland -- in charge of US policy toward Ukraine, according to testimony by current and former officials.

During the May meeting, Mulvaney put Energy Secretary Rick Perry, US special representative for Ukraine Kurt Volker and Sondland at the helm of US policy toward Ukraine, according to private testimony by George Kent, the State Department's deputy assistant secretary responsible for Ukraine matters. Mulvaney said Thursday he has never talked to Kent.

The news came as a surprise to US officials with more subject matter expertise on Ukraine, including those at the embassy.

"They revealed that decision at a meeting with Zelensky in Kiev, I believe on June 2," said Rep. Gerry Connolly, a Virginia Democrat who heard Kent's testimony. "And for some Americans from the embassy that was news to them."

Text messages between Sondland, Volker and the top US diplomat in Ukraine, Bill Taylor, would later reveal Sondland and Volker's coordination with Rudy Giuliani, Trump's personal attorney, and their efforts to pressure Ukraine to announce investigations into 2016 matters and Burisma, the company on whose board Hunter Biden sat. Taylor raised concerns that a quid pro quo was afoot: withholding a White House visit for the Ukrainian president and security aid until Ukraine committed to carrying out investigations.

At key junctures, officials elevated their concerns about the situation to their superiors and to lawyers in the White House counsel's office. That includes Hill, the top Russia adviser at the White House, acting at the direction of her boss, Bolton.

It's not clear where -- if anywhere -- those lawyers took the matter from there. As evidenced by the transcript of Trump's July 25 phone call, there was no reversing the influence that Giuliani and Sondland were having on the administration's Ukraine policy, despite the serious concerns being raised by senior officials.

After Trump's phone call, those same lawyers fielded additional complaints about the conversation itself, and took the unusual step of placing a transcript of the conversation in a highly secured server.

But those steps amounted to a culmination of long-simmering concerns, not the start of them. Even before the visit from the Ukrainians on July 10, Hill and other top officials were alarmed by the removal two months earlier of Marie Yovanovitch, the 33-year veteran of foreign service who'd been recalled from her ambassador post in Ukraine. Yovanovitch told lawmakers last week she'd been told Trump had lost faith in her even though she'd done nothing wrong.

Confirming existing fears

The early-July meeting at the White House -- designed to introduce members of the new Ukrainian administration to Washington -- only confirmed the existing fears that politics was running roughshod over professional diplomacy in Ukraine.

It was what happened after those talks that unnerved aides like Hill, according to testimony she delivered behind closed doors on Capitol Hill this week. During a session in a White House dining room after the official meeting ended, she said she recalled Sondland discussing investigations — a subject that was interpreted as a reference to the President's call for investigations into the Bidens.

She also recalled hearing Sondland mention Burisma -- the energy firm on whose board Hunter Biden served.

"She saw wrongdoing related to the Ukraine policy and reported it," one source familiar with her testimony said.

But even as Bolton and Hill sought to call attention to potential impropriety, Bolton drew up a scheduling request for a call between Trump and his Ukrainian counterpart.

The request was dated July 10, according to a senior White House official, the same day of that White House meeting that sparked concerns.

Trump's call with Zelensky would not take place until two weeks later -- on July 25 -- an unusual lag time between Bolton's scheduling request and the call, according to current and former White House officials.

The call was tentatively scheduled for July 20, but a senior White House official said it was delayed because Trump had a call scheduled with the Swedish prime minister to discuss the imprisonment of rapper A$AP Rocky, a top priority for Trump at the time.

As Trump was preparing to speak with Zelensky by phone in mid-July, officials throughout the government were warning of the dangers posed by his attempts to circumvent American diplomacy.

Some officials -- including Hill -- were skeptical of a phone call, believing it could be used to advance the political agenda pressed by Giuliani, but Perry and Sondland made the case for the call by focusing on the importance of Ukraine's energy independence from Russia and the prospects for increasing Ukraine's import of US natural gas, officials said.

But an official who reviewed Trump's briefing materials for his call with Zelensky said the materials focused on the new Ukrainian leaders' anti-corruption platform, not energy matters.

The call sheet Bolton generated to recommend scheduling the call focused "on Zelensky's potential to eliminate corruption in particular," one official said, and briefing materials talked up Zelensky's anti-corruption campaign platform.

In addition to scheduling issues with Trump's call to Sweden's prime minister on July 20, the call was pushed back five days to occur after parliamentary elections in Ukraine. One official said Bolton was also concerned that a call before the elections could signal that the US was attempting to improperly boost Zelensky's party, something Trump has not been afraid of doing with other countries' domestic politics.

The delay allowed Taylor, the US chargé d'affaires in Ukraine, to register his concerns about injecting politics into US-Ukraine ties.

"President Zelenskyy is sensitive about Ukraine being taken seriously, not merely as an instrument in Washington domestic, reelection politics," Taylor wrote in text messages.

Three hours later, Sondland replied to reassure him.

"Absolutely, but we need to get the conversation started and the relationship built, irrespective of the pretext," he wrote. "I am worried about the alternative."