A whistleblower complaint filed within the intelligence community is at the center of a House impeachment inquiry focused on President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE’s contacts with Ukraine.

The complaint accuses Trump of pressuring Ukrainian President Volodymyr Zelensky into investigating 2020 presidential candidate Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE and his son, and alleges that other close associates of Trump were involved in the effort.

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The controversy has intensified scrutiny on a cast of characters from both countries and raised further questions about the extent of Trump and his associates' dealings with Kyiv.

Here is a look at the key players facing questions in the wake of Trump's call with Zelensky.

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Giuliani, Trump's personal attorney, is accused of being a central figure in Trump’s push to get Ukraine to investigate Biden and his son over unfounded allegations of corruption. A White House memo of Trump’s July 25 call with Zelensky indicates that Trump asked the foreign leader to work with Giuliani as part of the investigation.

The whistleblower alleges that Giuliani traveled to Madrid after the call to meet with Andriy Yermak, a top adviser to Zelensky. The meeting was allegedly “a ‘direct follow-up’ to the President’s call with Mr. Zelensky about the ‘cases’ they had discussed," according to the whistleblower complaint.

Giuliani has publicly acknowledged lobbying the Ukrainian government to look into matters related to the Bidens. The House has subpoenaed Giuliani as part of their impeachment inquiry, though the attorney has suggested he won’t comply.

On Thursday, two men who aided Giuliani’s effort to investigate Biden were arrested on campaign finance violation charges. The allegations appear unrelated to Giuliani’s influence efforts in Ukraine.

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The complaint alleges that Barr was involved in the president’s effort, and a White House memo confirms that Trump asked Zelensky to work with the attorney general as part of the probe. However, the Justice Department has denied the charges.

“The Attorney General has not communicated with Ukraine -- on this or any other subject,” a spokeswoman for the Justice Department said last month.

The Justice Department has also said that it declined to mount an investigation into Trump’s July phone conversation with Zelensky because it did not violate campaign finance law.

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Pompeo, Trump's secretary of State, has admitted to being on the president’s July phone call with Zelensky, though has yet to comment on specifics surrounding the conversation. Asked about the discussion last week, Pompeo said that it regarded America’s policy toward Ukraine.

“Taking down the threat that Russia poses there in Ukraine. It was about helping the Ukrainians to get graft out and corruption outside of their government,” he said at the time.

House Democrats last month subpoenaed Pompeo for documents and scheduled depositions with five current and former members of the administration. Pompeo has dismissed their demands, claiming that the requests are an attempt to “intimidate” and “bully” the State Department. The State Department on Tuesday ordered Ambassador to the European Union Gordon Sondland Gordon SondlandGOP chairman vows to protect whistleblowers following Vindman retirement over 'bullying' Top Democrat slams Trump's new EU envoy: Not 'a political donor's part-time job' Trump names new EU envoy, filling post left vacant by impeachment witness Sondland MORE not to appear for a deposition.

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House Democrats on Thursday subpoenaed Energy Secretary Rick Perry for documents related to his potential involvement in Trump's effort to push Ukraine to investigate Joe Biden. A group of Democratic chairmen said that new "public reports have raised questions about any role" Perry may have played in conveying the president's message to Ukraine.

Perry has asserted that he was unaware of Trump's request for Ukraine to investigate the Bidens.

CNN reported earlier this week that Trump directed Perry and multiple State Department officials to talk with Giuliani after Zelensky requested an opportunity to meet with the president. The revelation appeared to highlight the authority Giuliani had with Trump in regards to Ukraine.

Trump also told House Republicans on a conference call last week that he called the Ukrainian president on July 25 at the urging of Perry, Axios reported.

Hunter Biden

Hunter Biden, son of former Vice President Joe Biden, served on the board of Ukrainian gas company Burisma between 2014 and 2019, reportedly making about $50,000 per month in the position. After Hunter Biden accepted the role in 2014, a spokeswoman for the vice president stated that the elder Biden "does not endorse any particular company and has no involvement with this company."

Trump's campaign to find dirt on Joe Biden mainly centers around the former vice president's work to oust the prosecutor, Viktor Shokin, looking into Hunter Biden.

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There is no evidence that suggests Hunter Biden was involved in any impropriety while working with Burisma. There is also no evidence that Joe Biden worked in his son's interests while pressuring Ukraine to oust Shokin.

“Hunter Biden cannot be responsible for violations of the management of Burisma that took place two years before his arrival,” Lutsenko said in an interview with The Washington Post. Trump in recent weeks has repeatedly attacked Joe Biden in the wake of the whistleblower complaint. Biden himself has denied any wrongdoing in Ukraine, and on Wednesday for the first time called for Trump to be impeached.

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Volker, the Trump administration's former envoy to Ukraine, abruptly resigned from his position as revelations surfaced about the administration’s dealings with the country. Volker is named in the whistleblower complaint as one of the officials who provided advice to Ukraine on “how to ‘navigate’” Trump’s request about the Bidens.

Ahead of testifying privately before Congress, Volker provided text messages that Democrats say showed Trump pushed Ukraine to investigate a political rival.

Some of the exchanges indicated that the administration’s position on a meeting between Trump and Zelensky was contingent upon the country looking into matters related to the 2016 election and the Bidens.

“Heard from the White House -- assuming President Z convinces trump he will investigate/'get to the bottom of what happened' in 2016, we will nail down date for visit to Washington," Volker said in a text to the Ukrainian adviser on the morning of July 25.

During his testimony, Volker emphasized that he found the allegations against Biden to lack credibility.

Gordan Sondland and William Taylor

Sondland, a wealthy hotelier who serves as the Ambassador to the European Union, worked with Volker to instruct Ukrainian officials on how to respond to Trump’s requests, according to the whistleblower complaint.

He’s come under heightened scrutiny over his text communications with administration officials. In an exchange with William Taylor, a top official in the U.S. Embassy in Ukraine, Sondland appears to reject Taylor’s concerns that Trump was seeking a foreign government to find dirt on a political opponent.

"As I said on the phone, I think it's crazy to withhold assistance for help with a political campaign," Taylor wrote in a Sept. 9 message, referring to military aid.

"Bill, I believe you are incorrect about President Trump's intentions," Sondland replied. "The president has been crystal 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.”

House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.) has said the committee would issue a subpoena for Sondland in light of the State Department's order to block his deposition. Schiff said that the official has important messages on a personal device.

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The Trump administration recalled Marie Yovanovitch, then the U.S. ambassador to Ukraine, from her post in May in a decision that Democrats denounced at the time as a "political hit job."

The whistleblower complaint alleges that Yovanovitch was recalled from Kiev because of claims made by a former top Ukrainian prosecutor. The State Department has called the accusations "an outright fabrication."

Trump's hostility towards the longtime U.S. diplomat was showcased in a White House memo of his July 25 call with Zelensky.

“The former ambassador from the United States, the woman, was bad news and the people she was dealing with in the Ukraine were bad news so I just want to let you know that,” Trump said.

Yovanovitch is slated to appear for a scheduled House deposition on Friday despite the White House's pledge to refuse to cooperate in the impeachment inquiry.

Lev Parnas and Igor Fruman

Lev Parnas and Igor Fruman, a pair of Florida-based businessman with ties to Giuliani, on Thursday were arrested on campaign finance violation charges. While the allegations are not related to Giuliani's work in Ukraine, the two have emerged as key figures in the ongoing controversy surrounding Giuliani's dealings in the country.

Parnas and Fruman reportedly helped introduce Giuliani to political figures in Ukraine as part of his effort to find dirt on the Bidens. Parnas told the Miami Herald that he used Giuliani as a vehicle to disclose allegations of wrongdoing about Biden and his son.

The two were among several individuals to receive documents and deposition requests as part of the House's impeachment inquiry. John Dowd, a former Trump attorney representing Parnas and Fruman, had said they would not comply.

Just days before their arrest, The Associated Press reported that the two were involved in an effort to persuade Ukraine into installing new management at the top of the country's state gas company. Those at the center of the push were reportedly planning to then use the new management to direct lucrative contracts to companies owned by allies of Trump's.

Viktor Shokin

Much of the controversy stems from the Trump administration’s push to investigate Biden’s effort as vice president to fire Shokin, a top prosecutor in Ukraine who had been probing a natural gas company whose board included Hunter Biden.

The Obama administration as well as several European nations pushed for Shokin’s ouster over his lack of action on corruption in the country. Joe Biden, who had taken a lead in the effort, traveled to Ukraine in December 2015 and said the U.S. would withhold $1 billion in aid if the country did not remove Shokin.

In March of 2016, an overwhelming majority of Ukrainian Parliament members voted to remove Shokin as prosecutor general, a decision that the EU hailed at the time.

Ukraine’s former President Petro Poroshenko has dismissed allegations of wrongdoing by Biden, saying the vice president never “raised any requests to open or close any concrete cases.” He also noted that Shokin’s dismissal came in response to “massive campaigns” from activists, politicians and the media.

Yuri Lutsenko

Lutsenko, who succeeded Shokin as Ukraine's prosecutor general, in September addressed allegations related to Hunter Biden’s work for Burisma, the Ukrainian gas company which he sat on the board of.

“From the perspective of Ukrainian legislation, he did not violate anything,” Lutsenko, who resigned from his post in August, told The Washington Post. “Hunter Biden cannot be responsible for violations of the management of Burisma that took place two years before his arrival."

The whistleblower notes in the complaint that Trump mentioned Lutskeno during the July 25 phone call with Zelensky and suggested that the leader keep him in his position. A White House readout of the call does not include references to Lutsenko by name.

Lutsenko told the Post that he was unaware of U.S. officials going to Ukraine to work on probes related to the Bidens.

“No American groups came to Ukraine for an investigation,” said Lutsenko, who served in his position between May 2016 and August.