As the House of Representatives prepares to hold open hearings in the impeachment inquiry against President Donald Trump next week, newly released transcripts of lawmakers’ closed-door interviews with government officials shed more light on Trump’s dealings with Ukraine and the central role his personal lawyer, Rudy Giuliani, played in U.S. relations with Ukraine.

On Monday, U.S. ambassador to the European Union, Gordon Sondland, offered a significant revision to his earlier testimony, saying he had delivered a message to Ukraine that the U.S. would not deliver aid pledged by Congress unless Ukrainian President Volodymyr Zelensky publicly stated that it would investigate the 2016 U.S. presidential election and a company that Hunter Biden, the son of former vice president Joe Biden, sat on the board of.

Other officials provided details in their testimonies on how a secondary channel for Ukraine policy was created and led by Giuliani, sidelining U.S. diplomats.

The impeachment inquiry centers on a July 25 phone call in which Trump asked Zelensky to investigate Biden and his son. A whistleblower report prompted lawmakers to investigate whether the U.S. withheld the aid from Ukraine in order to pressure them on matters that might benefit Trump politically.

Here’s what we learned this week:

Gordon Sondland, U.S. ambassador to the European Union

(Read Sondland’s full testimony.)

In a stark reversal this week, Sondland revised his earlier testimony to lawmakers. He said he now recalls relaying a kind of quid pro quo message to Ukraine, immediately following a meeting between Zelensky and Vice President Mike Pence.

He told a top Ukrainian aide that “resumption of U.S aid (money) would likely not occur until Ukraine provided the public anti-corruption statement that we had been discussing for many weeks.” The Trump administration had asked for a statement that Ukraine was investigating an energy company connected to the Bidens and the 2016 U.S. election.

Sondland defended Trump to lawmakers during his testimony. Sondland said he had “assumed” the aid and investigations were connected and that Trump repeated to him that he did not want a “quid pro quo.”

Fiona Hill, former Russia adviser for President Trump

(Read Hill’s full testimony.)

Hill recalled Sondland saying that acting White House Chief of Staff Mick Mulvaney had agreed Trump would meet with Ukraine’s president if they opened investigations into “the energy sector,” which she said was code for Burisma.

Hill also testified that then-National Security Adviser John Bolton thought such an arrangement was inappropriate.

The former adviser said she had received death threats and other targeted attacks similar to Marie Yovanovitch, who was forced out of her position as U.S. ambassador to Ukraine.

Hill testified that she thought the “most obvious explanation” for the attacks against Yovanovitch “seemed to be business dealings of individuals who wanted to improve their investment positions inside of Ukraine itself, and also to deflect away from the findings of” special counsel Robert Mueller’s report, as well as findings on Russian interference made by the Senate.

Alexander Vindman, director of European affairs at the National Security Council

(Read Vindman’s full testimony.)

Vindman is the only government official interviewed so far in the impeachment inquiry who listened to the call between Trump and Zelensky.

“I was concerned by the call. I did not think it was proper to demand that a foreign government investigate a U.S. citizen,” Vindman testified, later adding that the tone of the call was “dour.”

Vindman told lawmakers there was no doubt in his mind about what Trump wanted from the call — a probe led by Ukraine into political rival Biden and his son. Vindman said he alerted two of his superiors about his concerns.

“I realized that if Ukraine pursued an investigation into the Bidens and Burisma it would be interpreted as a partisan play, which undoubtedly would result in Ukraine losing the bipartisan support [in the U.S.] it has thus far maintained. This would undermine U.S. national security,” Vindman testified.

George Kent, deputy assistant secretary of state for European and Eurasian affairs

(Read Kent’s full testimony.)

Kent told lawmakers that Trump “wanted nothing less than President Zelensky to go to the microphone and say” three words: investigations, Biden and Clinton. “Clinton,” as in Hillary Clinton, was shorthand for the 2016 election, Kent said.

He also testified that he was sidelined from official U.S. policy on Ukraine when a second channel of decision-making rose in late summer, with the influence of partisan figures such as President Donald Trump’s attorney Giuliani overtaking and even blocking the work of officials and government staff.

William Taylor, charge d’affaires at the U.S. Embassy in Kiev

(Read Taylor’s full testimony.)

Taylor corroborated Kent’s testimony about a second channel opening up on Ukraine policy.

Along with Kent, Taylor relayed that an official with the Office of Management and Budget told them and others that the president had ordered a hold on to around $400 million in aid that Congress had approved for Ukraine.

Taylor said he understood that the money would not be given until Ukraine’s president committed to investigating Burisma, the energy company that Hunter Biden sat on the board of. “I understood the reason for investigating Burisma was to cast Vice President Biden in a bad light,” Taylor said.

Neither Taylor nor Kent personally heard the president make orders related to Ukraine aid or requiring Ukraine to launch investigations. Both heard Ambassador Sondland make that connection.

Kurt Volker, former U.S. special representative for Ukraine

(Read Volker’s full testimony.)

In text messages Volker gave to investigators, U.S. diplomats discussed the Trump administration’s interest in getting Ukraine to investigate potential 2016 election interference and Burisma.

Both Sondland and Volker testified that, even though they officially led Ukraine policy, they initially struggled to determine why the United States had frozen around $400 million in aid money to Ukraine.

Marie Yovanovitch, former U.S. ambassador to Ukraine

(Read Yovanovitch’s full testimony.)

Yovanovitch testified that Ukrainians had warned her that the president’s personal attorney, Giuliani, was trying to have her removed from her post.

Yovanovitch, who was ousted in May, believed Giuliani and his associates were motivated by business and political deals they or their clients hoped to make.

In response to a question from Rep. Tom Malinowski, D-N.J., Yovanovitch confirmed that the “parallel policy” on Ukraine led by Giulianni overtook U.S. policy made through traditional channels.

Michael McKinley, former senior adviser to Secretary of State Mike Pompeo

(Read McKinley’s full testimony.)

McKinley asked Secretary of State Mike Pompeo three times to defend Yovanovitch. Pompeo refused, and McKinley resigned after more than three decades as a diplomat.

The PBS Newshour’s Lisa Desjardins, Ali Rogin, Nick Schifrin and Mike Melia contributed to this report.

Subscribe to the PBS NewsHour’s impeachment newsletter.