Here are all the major players in the Trump-Ukraine scandal

Zelensky, a former comedian and TV star, was elected as Ukraine's president in April after he ran on an anticorruption platform.

Trump pressured Zelensky to probe both CrowdStrike, a cybersecurity firm retained by the DNC to investigate Russia's hack of its servers in 2016, and the Bidens during his July phone call.

Ukraine is heavily reliant on US military aid in its fight with Russian-backed militants.

Zelensky has tried to distance himself from the ongoing controversy engulfing at the White House.

At the United Nations General Assembly meeting in late September, Zelensky denied that he felt pressure from Trump to investigate the Bidens, and emphasized that he did not want to be involved in American elections.

Read more: Meet Volodymyr Zelensky, Ukraine's comedian turned president who's at the center of the scandal threatening Trump's presidency

As former President Barack Obama's top deputy, Biden served as the administration's point-person for US-Ukraine policy and relationships.

When he called for Shokin to be fired, Biden represented the US's official position on the matter, one that was shared by many other Western governments and anticorruption activists in Ukraine, according to The Associated Press.

However, Trump and Giuliani allege that Biden pushed for Shokin's ouster because he wanted to stymie the investigation into Burisma.

But there's a loophole in those claims. Government officials and Ukrainian anticorruption advocates say Shokin had hampered the investigation into Burisma long before Biden even stepped into the picture, according to the Wall Street Journal.

Biden has called Trump's push to have him investigated by a foreign power disturbing and said it shows Trump is unfit for office.

Read more: Trump brought up Joe Biden during a June phone call with Chinese President Xi Jinping

Hunter Biden, Joe Biden's eldest son, served on the board of Ukrainian oil and gas company Bursima Holdings from 2014 to 2019 providing legal advice and receiving a reported salary of around $50,000 a month.

Burisma was the subject of an investigation from the Ukrainian prosecutor general's office into whether its founder Mykola Zlochevsky engaged in tax evasion, money laundering, and corruption.

When former Ukrainian prosecutor general Viktor Shokin assumed the role in 2015, the investigation into Burisma was largely dormant, and there is no evidence that Hunter was personally involved in any wrongdoing.

Read more: A Ukraine gas company tied to Joe Biden's son is at the center of the Trump-whistleblower scandal

Giuliani is Trump's personal lawyer and not a government official. He has functioned largely as Trump's key envoy in the Ukraine controversy, and he used his connections to Trump and other US government officials to push Ukrainian officials to investigate the Bidens.

The whistleblower complaint said — and Giuliani has acknowledged — that Trump and State Department officials enlisted him to work with the Ukrainian government to investigate the Bidens.

Giuliani was mentioned 31 times in the complaint and described as a "central figure" in the administration's efforts to encourage Ukraine to investigate the Bidens, which included serving as an "envoy" on the matter and meeting with an aide to Zelensky, Andriy Yermak.

Giuliani, meanwhile, has defended himself by saying he only got involved because Volker asked him to.

Read more: How Rudy Giuliani went from a respected federal prosecutor and beloved NYC mayor to Trump's bag man

Pence wasn't a party to the call, but has still been roped into the Ukraine saga.

Trump used Pence to convey to Zelensky that the US would withhold military aid to the country while demanding that it aggressively investigate corruption, the Washington Post reported.

The Ukrainian government officials understood that Trump's demand to investigate corruption was connected to his desire for them to look into former Vice President Joe Biden and his son ahead of the 2020 election, the Post said.

Trump's direction to Pence came shortly after his July 25 phone call with Zelensky.

Pence addressed the issue of corruption in Ukraine during a diplomatic trip to Poland in August, before the public learned of the whistleblower's complaint and after it was reported that the US was withholding aid from Ukraine.

The day after he met with Zelensky during the trip, Pence was asked about the substance of their conversation. He said they did not discuss Biden but said Trump had asked him to convey to Zelensky that the US has "great concerns about issues of corruption."

Pence said that before investing more taxpayer money in Ukraine, "the president wants to be assured that those resources are truly making their way to the kind of investments that will contribute to security and stability in Ukraine, and that's an expectation the American people have and the president has expressed very clearly."

Pence received a transcript of the phone call before his diplomatic trip, but officials close to him said he did not review it closely and had no knowledge that Trump brought up the Bidens during the conversation.

Pompeo, Trump's former CIA director, is his most trusted official on foreign policy matters.

After initially denying that he had knowledge of Trump's July 25 call with Zelensky, Pompeo later admitted to being a party to the conversation.

In addition to misrepresenting his involvement in the Trump-Zelensky call, Pompeo is also accused of obstructing the congressional investigation into the matter.

In September, Pompeo sent a letter to the House Intelligence, Foreign Affairs, and Oversight committees, telling them their requests to depose department officials connected to the whistleblower's complaint are "not feasible" and "do not provide adequate time for the Department and its employees to prepare" for testimony.

In turn, Democratic lawmakers accused Pompeo of "intimidating Department witnesses" from testifying and warned that it "will constitute evidence of obstruction of the impeachment inquiry."

Pompeo has dismissed the impeachment inquiry as a "silly gotcha game," the Associated Press reported.

"When I talk to your foreign minister he pressures me all the time," Pompeo said to a Greek reporter in Athens. "It is totally appropriate. Nations do this. Nations work together. They say 'Boy, goodness gracious if you can help me with X, we'll help you achieve Y.' This is what partnerships do. It's win-win."

Read more: Mike Pompeo made at least 4 significantly misleading statements about his role in the Trump-Ukraine phone call

The White House's notes of Trump's call with Zelensky show that Trump encouraged Zelensky to with both Giuliani and Barr to investigate the Bidens and the origins of the Russia probe.

Barr was "surprised and angry to discover he had been lumped in with Giuliani" in the call, the Associated Press reported.

Later, the Washington Post reported that Barr's involvement went deeper than first met the eye — and that he had actively solicited foreign help in a probe about the origins of US investigations into Russian interference in the 2016 election.

The report said Barr had already made overtures to British intelligence officials and traveled to Italy, where he and John Durham, the US attorney in charge of investigating the Russia investigation's origins, asked senior Italian government officials to assist with Durham's work. The report said it was not Barr's first trip to Italy to meet with intelligence officials.

Barr also reportedly called Trump and encouraged him to tell Rudy Giuliani to tone down his wild TV appearances, according to The Wall Street Journal.

Read more: 'Pure insanity': Intelligence veterans are floored by Barr's 'off the books' overtures to foreign officials about the Russia probe

The president also implicated US Secretary of Energy Rick Perry in the Ukraine saga, claiming recently that Perry had encouraged him to call Zelensky regarding Burisma.

"Not a lot of people know this but, I didn't even want to make the call," Trump said on a call with House Republicans, a source familiar with the comments told Axios. "The only reason I made the call was because Rick asked me to. Something about an LNG [liquefied natural gas] plant."

Following Axios' report, Perry acknowledged that he "absolutely" encouraged the president to call Zelensky "multiple times," but not about the Bidens.

Read more: Trump is reportedly blaming Rick Perry for his infamous call with the Ukrainian president

Schiff, an experienced committee chair and ranking member, is taking the lead on the impeachment inquiry, calling witnesses and subpoenaing documents as part of the investigation.

So far, his committee has heard testimony from Acting Director of National Intelligence Joseph Maguire and closed-door testimony from Volker and Atkinson.

The intelligence committee has also issued subpoenas to Giuliani and Pompeo.

It recently surfaced that the whistleblower approached a committee aide with their concerns about the call before filing an official complaint. The whistleblower gave the aide a "vague" account of their allegations, The New York Times reported, and the aide followed committee rules and advised the whistleblower to hire a lawyer and file an official complaint.

Read more: 2 key facts refute Trump's conspiracy theories about the Ukraine scandal

Volker worked in an unpaid, volunteer capacity as the US' Special Representative to Ukraine. But he got more than he bargained for when he was swept up in the Ukraine scandal after being named in the whistleblower's complaint.

According to text messages turned over to Congress, Volker played an active role in setting up communications and meetings between Giuliani and Yermak, Zelensky's aide.

In written testimony submitted to the House Intelligence Committee, Volker emphasized that "at no time was I aware of or took part in an effort to urge Ukraine to investigate former Vice President Biden."

But the text messages paint a different picture and show Volker was aware of Trump's intentions from the get-go and did not raise any concerns about the matter.

"Heard from White House — assuming President Z convinces trump (sic) he will investigate/'get to the bottom of what happened' in 2016, we will nail down date for visit to Washington," Volker wrote in an August 30 text message to fellow diplomat Bil Taylor, seemingly implying that a meeting between Zelensky and Trump would be contingent on investigations.

Volker has since resigned from his position as Special Envoy to Ukraine and from the McCain Institute.

Read more: Newly revealed text messages show Trump diplomats' internal turmoil over his pressure on Ukraine

Bill Taylor, a career diplomat, served as the US ambassador to Ukraine under George W. Bush's administration from 2006 to 2009, and was appointed as the chargé d'affaires for the US embassy in Ukraine earlier this year.

In the text messages submitted to Congress, Taylor repeatedly raised concerns that Trump was leveraging a meeting with Zelensky and military aid in exchange for "investigations."

After Trump canceled a planned visit to Warsaw, Poland, on August 30, where he was set to meet with Zelensky, Taylor "sought clarification" on the conditions for the Ukrainian president to visit the White House the next day.

"Are we now saying that security assistance and [White House] meeting are conditioned on investigations?" Taylor wrote to Sondland in a September 1, exchange.

In another exchange on September 9, Taylor wrote to Sondland saying that Trump halting military aid to Ukraine has "shaken their faith in us," which he said was his "nightmare scenario."

Taylor later added that he thought that Trump's decision to "withhold security assistance for help with a political campaign" is "crazy."

Sondland made a fortune as a successful hotel executive and entrepreneur in the Pacific Northwest and became a generous donor to Republican politicians — which included giving $1 million to Trump's 2017 inaugural fund.

In 2018, Trump nominated Sondland to be the US' ambassador to the European Union.

Giuliani, Volker, and Sondland all exchanged messages regarding Giuliani's communications with Ukranian officials.

In one exchange that took place on July 21, Taylor texted Sondland saying Zelensky was "sensitive about Ukraine being taken seriously, not merely an instrument in Washington domestic, reelection politics."

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

After Taylor expressed concern in a September 1 exchange that withholding the funds in exchange for an investigation was "crazy," Sondland responded, "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," Sondland added.

Read more: Meet the latest actor in the whirlwind Ukraine inquiry: hotelier turned ambassador Gordon Sondland

Masha Yovanovitch, a veteran diplomat who worked in the US Foreign Service for over three decades, served as the US ambassador to Ukraine from 2016 to May of 2019.

The Wall Street Journal reported that Trump pushed her out of the administration when people close to him — particularly Giuliani — lamented that she displayed an "anti-Trump bias" and got in the way of his efforts to persuade the Ukranian government to open the investigations Trump wanted them to pursue.

Now, Yovanovitch is poised to become a key witness in the House's impeachment inquiry. She is scheduled to appear for a closed-door joint testimony session before the House Intelligence, Oversight, and Foreign Affairs Committees on October 11.

Read more: Trump reportedly personally ordered the removal of ambassador to Ukraine after people said she was impeding Biden investigations

As the intelligence community's chief watchdog, Atkinson was the one who first received the whistleblower complaint and conveyed it to Maguire.

Atkinson confirmed that the whistleblower complaint was "credible" and rose to the level of an "urgent concern" in a letter to Maguire.

He determined that in addition to potentially violating campaign finance laws, Trump's conduct could also open him up to foreign blackmail.

Specifically, he wrote to Maguire: ".... alleged conduct by a senior US public official to seek foreign assistance to interfere in or influence a federal election would constitute a 'serious or flagrant problem'" and would also "potentially expose such a US public official to serious national security and counterintelligence risks with respect to foreign intelligence services aware of such alleged conduct."

Atkinson and Maguire referred the complaint to the Justice Department for criminal investigation, but the department determined that Trump's request to Zelensky did not constitute a "thing of value" and therefore did not violate campaign finance laws.

Maguire has served as acting DNI since August and was one of the first officials to become aware of the complaint.

He was criticized for taking the complaint first to the White House and not to Congress. He appeared in open session before the House Intelligence Committee on September 26. Maguire said he contacted the White House because the president was the subject of the complaint and that it could therefore raise issues of executive privilege.

The White House and Justice Department advised Maguire to withhold the document from Congress, prompting a showdown with Schiff's committee before Maguire eventually forwarded the complaint to lawmakers.

In his opening statement at his hearing, Maguire said, "I want to stress I believe the whistle-blower and the inspector general have acted in good faith throughout. I have every reason to believe that they have done everything by the book and followed the law."

Maguire also confirmed that the substance of the whistleblower complaint is "in alignment" with the White House's memo of Trump and Zelensky's July call.

Read more: Acting DNI Joseph Maguire undermined the GOP's entire argument against the whistleblower in one sentence

Viktor Shokin was the controversial Ukrainian prosecutor-general ousted at the beginning of 2016 over allegations that he turned a blind eye to pervasive corruption.

Trump and Giuliani accused Biden of trying to protect Hunter by calling for Shokin's ouster, but as previously noted, that narrative doesn't hold up under scrutiny.

Shokin wasn't even actively investigating Burisma at the time, and many foreign government leaders and officials — including Biden — urged Shokin to be fired based on his ineffectiveness as a prosecutor, not because he posed a threat to Burisma.

Western diplomats also say he essentially shut down one such investigation into Burisma's founder in the UK by refusing to cooperate with authorities. Bloomberg also reported that the Burisma investigation was largely dormant when Biden called for Shokin to be fired.

Read more: There's a glaring loophole in Trump and Giuliani's allegations of corruption against Joe Biden

Yuriy Lutensko, a former member of parliament, served as Ukraine's prosecutor general from 2016 until August of 2019 after Shokin's ouster.

Earlier this year Lutsenko clearly said he had no evidence of wrongdoing by Joe or Hunter Biden.

I do not want Ukraine to again be the subject of US presidential elections," Lutsenko said. "Hunter Biden did not violate any Ukrainian laws ... at least as of now, we do not see any wrongdoing."

Lutensko is now the subject of an investigation by Ukrainian officials as to whether he used the power of his office to "provide cover" for illegal gambling activities, USA Today and the Guardian recently reported.

• Read more:
  
  Newly revealed text messages show Trump diplomats' internal turmoil over his pressure on Ukraine
  
  2 top Trump deputies drafted a statement for Ukraine's president committing him to pursuing political investigations for Trump
  
  The 2nd Trump whistleblower is an IRS official who alleges a Treasury Department political appointee tried to interfere with Trump's tax audit
  
  As Trump rages about corruption, his State Department has proposed drastically cutting its anticorruption program