(CNN) The fast-moving scandal involving President Donald Trump and allegations that he pressured Ukrainian President Volodymyr Zelensky to investigate his political rival Joe Biden and his son, Hunter, has upended politics in Washington, DC, and evolved into a full-blown impeachment investigation. There is no evidence that either Biden did anything wrong.

For an up-to-the-minute look at who has testified and who has not, go to the impeachment tracker . To meet the players, read on.

The whistleblower

An anonymous government worker - with access to White House officials - who filed an official complaint against the President claiming that multiple officials had concerns that Trump was using his public office to seek personal political gain from a foreign power not just through his July 25 phone conversation with Zelensky but through emissaries including Trump's lawyer, Rudy Giuliani, as well as, possibly, Attorney General Bill Barr. The complaint also alleges a coverup by White House staffers who sought to bury Trump's conversation with the Ukrainian President by placing a rough transcript of it in a computer system reserved for highly classified material.

Donald Trump, President of the United States

Just as he was trying to move on from allegations his campaign colluded with Russians against Hillary Clinton in 2016, Trump was ramping up efforts to get Ukraine to stir up trouble for his prospective 2020 challenger Biden. He used official channels to ask the Ukrainian President to look into the Biden family, openly asking Zelensky for a favor during a phone call on July 25 and then suggested his attorney meet with Ukrainian officials. In the July 25 phone call, the Ukrainian President mentioned plans to buy US-made Javelin missiles -- which he needs to help guard against potential Russian provocations. At the very same time, Trump was sitting on nearly $400 million in aid to Ukraine, which he has argued was on hold while he leaned on European countries to give more to Ukraine.

Volodymyr Zelensky, President of Ukraine

A comedian turned politician, Zelensky's job before being elected the President of Ukraine was playing the President of Ukraine on television. His country is in the midst of a years-long standoff with Russia over Crimea, an area Russia invaded and annexed in 2014. Zelensky is also dealing with a war against pro-Russian separatists. But his intentions -- and whether he is pro-Russian or pro-West -- have been something both sides have been trying to determine. His desire for military aid from the US is unquestioned. He flattered Trump during their phone call in July and promised a fair look at the Biden family after Trump asked for an investigation. But Zelensky clearly does not want to get involved in US politics , and said after the White House released its transcript of the July 25 call that he didn't think his side would be included.

Rudy Giuliani, Trump attorney

The former New York mayor, now the President's personal attorney and staunch defender on cable news, is at the center of the Ukraine scandal . He has said he went to Ukraine because was trying to undermine the beginnings of the Russia investigation and protect Trump's former campaign chairman Paul Manafort, who is currently in prison for tax fraud linked to his business dealings in Ukraine. Giuliani has said he then pivoted after learning about claims concerning Joe Biden's actions as Vice President, including his public calls in 2016 for a prosecutor in Ukraine to be fired. Giuliani claims that was improper because, at the time, Biden's son sat on the board of a Ukrainian company that had once been under investigation in Ukraine. After pushing the story about Biden for months, Giuliani ultimately met with a top aide to Zelensky in Madrid, days after the July 25 phone call where Trump asked Zelensky to hear Giuliani out.

William Barr, US attorney general

Barr took over as head of the Department of Justice in February 2019, and oversaw the official release of the Mueller report summing up the government's investigation into claims that Russia engaged with the Trump campaign to tamper with the 2016 election. Trump asked the Ukrainian President to talk with Barr as part of an investigation he said Barr was conducting. Barr has remained quiet about any involvement in Ukraine, but Democrats have said they expect him to be called to testify

Mike Pompeo, US Secretary of state

Mike Pence, vice president of the US

When Trump canceled a September trip to Poland during which he was supposed to meet with Zelensky, Pence went instead. The vice president told reporters after the meeting that he did not discuss Biden, but that he talked to Zelensky in great detail about Trump's interest in rooting out corruption in Ukraine, and also about US aid to Ukraine.

Joseph Maguire, acting director of national intelligence

A retired vice admiral and former Navy SEAL, Maguire stepped into his role when former DNI Dan Coats resigned in August. Maguire initially withheld the whistleblower complaint from Congress because his attorneys counseled him that Trump was not part of the intelligence community. Weeks into the job, he's already been called to testify before Congress over the Ukraine scandal. In that congressional testimony he defended the whistleblower for coming forward.

Pat Cipollone, White House counsel

Alongside the allegations about Trump's effort to influence the Ukrainian President against Biden are the whistleblower's allegation that White House lawyers sought to cover things up by burying the transcript of the call in a server reserved for highly classified information. Cipollone, as White House counsel, was also involved in the recommendation that Maguire should withhold the whistleblower complaint from Congress, according to the Washington Post

Adam Schiff, House Intelligence Committee Chairman

A former federal prosecutor, Schiff is a confidant of House Speaker Nancy Pelosi. She has designated the California congressman to take the lead in the Democrats' inquiry into the whistleblower complaint. He is a frequent target of Trump, who refers to him as "liddle" Adam Schiff.

Nancy Pelosi, Speaker of the US House of Representatives

Pelosi is in her second stint as the top elected Democrat in the country, and nothing will move forward on impeachment without her support. After months of slowing calls by Democrats for impeachment proceedings over the findings of the Mueller report, Pelosi quickly moved to change course and begin official impeachment proceedings after the substance of Trump's Ukraine actions as detailed in the whistleblower complaint became clear.

Devin Nunes, House Intelligence Committee Ranking Member

If Schiff is the prosecutor, Nunes -- also from California -- will play the role of Trump's defender. Long an apologist for the President on the Russia investigation, Nunes has pivoted to defending Trump on Ukraine. With his seat next to Schiff, he will play an important role.

Jim Jordan, House Oversight Committee Ranking Member

Jordan is more vocal and more visible than Nunes and never wears a jacket. He's all over transcripts of closed-door testimony speaking up on Trump's behalf and peppering witnesses with questions.

Yuriy Lutsenko, former Ukrainian General Prosecutor

A holdover from the administration that preceded Zelensky in Ukraine, Lutsenko was Giuliani's original target to influence toward an investigation of the Bidens and Burisma. Lutsenko said in May he had looked to reanimate the investigation, but also that there was no evidence of any wrongdoing by Hunter Biden. His comments to a reporter for The Hill that there should be an investigation into whether Ukrainians had meddled in the 2016 US election may have helped fuel Giuliani's focus on Ukraine. He resigned in August.

Viktor Shokin, former Ukraine prosecutor general

Shokin was named Ukraine's top prosecutor under former Ukrainian President Petro Poroshenko in 2015 and began facing criticism for what was seen as an unwillingness to prosecute elite corruption. Biden, who handled Ukraine issues for the Obama administration, put public and private pressure on the Ukraine government to fire Shokin. The Obama administration threatened to withhold $1 billion in loan guarantees unless Poroshenko took action. Trump has seized on this to accuse Biden of wrongdoing -- despite no evidence of this -- and the international calls for Shokin's firing at the time. The Ukraine legislature voted to oust Shokin in March 2016. Biden's last visit to Ukraine before Shokin's firing was in December 2015, though he held a phone call with Poroshenko before the dismissal.

Lev Parnas and Igor Fruman

Lev Parnas (left) and Igor Fruman

Soviet-born American associates of Giuliani who helped him in Ukraine but also had business interests there, Fruman and Parnas were arrested on campaign finance violation allegations in the US. It has since been reported by CNN that federal authorities are scrutinizing their business ties to Giuliani. Their alleged campaign finance misdeeds include making campaign contributions using so-called "straw donors." They donated $325,000 to a Trump-aligned super PAC in 2018 and have put pictures of themselves with the President on social media. The super PAC, America First ACTION, said in a statement it never spent the money.

Vladimir Putin, President of Russia

Putin is not directly related to the whistleblower complaint, but his shadow looms over this entire story and generally over Trump's presidency. The reason Zelensky and Ukraine want the military aid is their standoff with Russia over Russia's 2014 annexation of Crimea.

Andriy Yermak, aide to Ukrainian President

A top adviser to Zelensky, Yermak met with Giuliani in Madrid a week after Trump's call with Zelensky. According to the whistleblower complaint, various US officials said this meeting was a "direct follow-up" to the July 25th call. He's also the aide who US Ambassador to the EU Gordon Sondland spoke with on the sidelines of a Sept. 1 meeting between Pence and Zelensky. Sondland testified that he told Yermak that US security assistance was conditional on the investigations.

T. Ulrich Brechbuhl, State Department Counselor

A West Point classmate and business partner of Pompeo, Brechbuhl is mentioned in the complaint as having listened to Trump's call with Zelensky as it occurred. The State Department has denied he listened in.

Kurt Volker, US Special Representative for Ukraine Negotiations

Formerly the administration's point person on Ukraine, Volker is a key figure in this story. He's mentioned in the whistleblower complaint as trying to advise Ukrainian officials on how to deal with Trump and Giuliani. It was Volker who apparently set up the meeting between Giuliani and Yermak, Zelensky's aide. He was essentially a volunteer, however, and still works for BGR group, a Washington lobbying firm that has represented the government of Ukraine. Volker resigned his position and was the first witness called by House investigators in their impeachment probe. Text messages he gave three House committees showed concern among diplomats that the Trump administration was withholding funding over Trump's demands for an investigation of his political rivals.

Gordon Sondland, United States Ambassador to the EU

William B. Taylor, chargé d'affaires for US Embassy in Ukraine

Taylor, the top US diplomat in Ukraine, was intimately involved in talks between Volker, Sondland and aides of Zelensky. He repeatedly raised concerns in text messages that the administration was withholding funding in exchange for an investigation of Trump's political rivals. He was ambassador to Ukraine during the George W. Bush administration and was brought back as the top official there after the recall of Yovanovitch. In congressional testimony, he detailed for impeachment investigators how he came to feel that Trump was holding up security aid to force the Ukrainians to launch investigations into the beginnings of the Mueller probe and also Burisma.

Marie Yovanovitch, former US Ambassador to Ukraine

After complaints by Lutsenko, the Ukrainian prosecutor, and Republicans in the US, Yovanovitch was recalled from her post. A career member of the foreign service who has served in ambassadorships under three presidents, she was sworn in as ambassador to Ukraine in August 2016 and recalled in May 2019, prompting Democrats to accuse the White House of a " political hit job ." In the phone call with Zelensky, Trump disparaged her, calling her "bad news" and saying, "she's going to go through some things." In damning testimony on Capitol Hill , Yovanovitch said it was her understanding she was targeted by Giuliani and she issued a dire warning about diplomacy during the Trump administration.

David Holmes, official in US embassy in Ukraine

A career foreign service officer and top official at the US embassy, Holmes testified he overheard Sondland discuss investigations in a loud phone conversation with Trump the day after Trump and Zelensky's phone call and while Sondland was in a restaurant in Kiev.

George Kent, Deputy Assistant Secretary of State

career foreign service officer , Kent is now the top policy official for Eurasia, including Ukraine. He was number two in the US embassy to Ukraine from 2015 to 2018, so his experience there spans time under Trump, as well as when then-Vice President Joe Biden was pressuring Ukraine to do more to combat corruption. He told impeachment investigators that Trump associates made baseless claims against Yovanovitch. He also said he raised some concerns in 2015 about Hunter Biden's position on the Burisma board.

Fiona Hill, former top White House Russia adviser

In congressional testimony she deconstructed the idea that Ukraine interfered in the 2016 election as Trump and other Republicans have alleged. A notable critic of Russian President Vladimir Putin, Hill officially resigned from her position with the National Security Council as Trump's top Russia policy adviser in August. She is mentioned in text messages given to House Democrats as part of the impeachment inquiry by former US Special Envoy for Ukraine Kurt Volker. She told investigators that former National Security Adviser John Bolton was suspicious of why aid was being held up and that he referred to shadow diplomacy being conducted in Ukraine as a kind of "drug deal." In congressional testimony she deconstructed the idea that Ukraine interfered in the 2016 election as Trump and other Republicans have alleged.

Tim Morrison, Russia expert on National Security Counsel

Morrison succeeded Hill at the NSC, but is already planning to leave. He's mentioned in text messages as a key contact at the White House for Sondland, Volker and Taylor. But he told impeachment investigators he tried to figure out whether Trump told Sondland he wanted Ukraine to investigate the Bidens. He's among a number of officials who worried Giuliani and others were pushing a shadow foreign policy.

Lt. Colonel Alexander Vindman, Ukraine expert on National Security Counsel

Vindman was on the call between Trump and Zelensky and he told impeachment investigators he raised concerns afterward with the top lawyer on the NSC. He also testified the official transcript of the Trump-Zelensky call omitted some key words . Vindman was born in Ukraine when it was part of the Soviet Union and fled to the US as a child, which led Trump and some Republicans to question his patriotism, despite his war service.

Jennifer Williams, adviser to Vice President Mike Pence

A longtime State Department staffer, Williams is detailed to Pence's office as special adviser on European and Russian affairs and was one of two Pence aides on the July 25 call between Trump and Ukrainian President Volodymyr Zelensky. The other was Keith Kellogg, the vice president's national security adviser. She was concerned about the call but did not inform her superiors about it.

David Hale, under secretary of state for political affairs

Hale is the third highest-ranking official at the State Department. He told House impeachment investigators that Secretary of State Mike Pompeo was reluctant to defend Ukraine ambassador Marie Yovanovitch because of concerns it would hurt efforts to get Ukraine military aid.

Philip Reeker, assistant secretary of state for European and Eurasian affairs

A career foreign service officer, Reeker testified that he didn't know about the push for Ukraine to undertake investigations.

Michael McKinley, former State Department senior adviser

A former top aide to Secretary of State Mike Pompeo, McKinley told lawmakers he quit his post because he didn't feel the department was sticking up for Yovanovitch. He said he was also concerned US political objectives were being thrust on diplomats

John Eisenberg, a deputy White House counsel

The first person at the White House notified about concerns ultimately raised by the whistleblower. He and the intelligence agency's general counsel contacted the Justice Department together, according to The New York Times . He's the top lawyer on the National Security Counsel, the person to whom Vindman raised concerns about the call between Trump and Zelensky and also the person who directed the call transcript be placed in a more secure server.

John Bolton, former National Security Adviser

Bolton left the White House in a huff in September . Hill testified he was leery of Giuliani's efforts and also of the holdup in aid to Ukraine. She said he referred to Giuliani as a "hand grenade" and the shadow diplomacy operation as a "drug deal."

Charles Kupperman, former deputy national security adviser

A former aide who shares an attorney with former National Security Advisor John Bolton, Kupperman was on the call between Trump and Zelensky. But he's since left the White House. He asked the courts to weigh in on whether he should respect a congressional subpoena or a White House claim that his testimony is privileged even though he's no longer at the White House.

Mick Mulvaney, acting White House chief of staff

A key figure at the White House, Mulvaney was integral in holding up funding to Ukraine. He argued to reporters that, yes, essentially, there was a quid pro quo, but that's totally okay in politics and diplomacy. He later clarified the comments . Along with most White House officials, he has refused to testify on Capitol Hill.

Robert Blair, a Mulvaney aide at the White House

Blair was on the call between Trump and Zelensky. He ignored a subpoena to testify.

Brian McCormack, OMB official focused on energy

A former chief of staff to Energy Secretary Rick Perry, McCormick now works at the OMB and could answer questions about aid to Ukraine. He did not comply with a Congressional subpoena.

Wells Griffith, White House adviser

Another former Perry aide, Griffith now works on the National Security Council.

Rick Perry, Secretary of Energy

Perry has announced his intention to resign, but he plays an important role in the Ukraine investigation. He traveled to Ukraine to meet the new president and he was deeply involved in US policy there, along with Volker and Sondland. For Perry, this also included efforts to push Ukraine to change leadership at Naftogaz, Ukraine's geopolitically important state-owned oil and gas company. That goal was shared by Giuliani's clients Parnas and Fruman.

Catherine Croft, State Department Ukraine expert

A former aide to Volker, Croft testified before impeachment investigators about efforts by allies of Rudy Giuliani, including former congressional powerhouse Bob Livingston, to have Yovanovitch recalled from Kiev.

Michael Ellis, White House lawyer

Eisenberg's deputy who might know how his boss handled concerns raised about the call between Trump and Zelensky and why it was placed in a more secure server.

Laura Cooper, Deputy Assistant Secretary of Defense

A top Pentagon career official overseeing Ukraine policy, Cooper has been a vocal advocate for US support for Ukraine in the face of the threat from Russia. She was asked for her understanding of why the security funding was delayed.

John Durham, US Attorney for District of Connecticut

Mentioned in the whistleblower complaint because he was tapped by Barr to lead a probe into the beginnings of the Russia investigation and, according to the DOJ, was investigating Ukraine.

Michael Atkinson, Inspector General of the Intelligence Community

As ICIG, Atkinson was the first to receive the whistleblower complaint . He informed Congress that it was initially being withheld from the House Intelligence committee despite his determination that it represented an "urgent concern." He told the committee he was at an "impasse" with Maguire over the complaint and whether Congress should be informed about its existence or its contents. He was appointed to his position by Trump after many years at the Department of Justice.

Stephen Engel, Director, Office of Legal Counsel

A top attorney at the Department of Justice, it was Engel's job to review whether the whistleblower complaint was an "urgent concern" from the perspective of the DOJ. He recommended keeping the complaint at the DOJ for a possible criminal probe instead of sending to Congress, as the law would seem to require. That set up the showdown between Atkinson, the inspector general and Maguire, the acting DNI.

General counsel at an intelligence agency

Even before the whistleblower filed the complaint, the White House was aware of the whistleblower's concerns because an intelligence agency's general counsel notified lawyers at the Justice Department and the White House after the whistleblower flagged them through an anonymous process, according to The New York Times

John Demers, head of the Justice Department's national security division; Jeffrey Rosen, deputy attorney general; Brian Benczkowski, head of DOJ criminal division

Demers was the first attorney at the DOJ notified of concerns about Trump's conduct raised by the whistleblower and discussed it with colleagues Rosen and Benczkowski at Department of Justice, according to The New York Times . It's not clear when or how Barr was notified.

Andrew Bakaj and Mark Zaid, attorneys for the whistleblower

Bakaj worked in the CIA inspector general's office before being forced out in 2014. A subsequent report by the Department of Homeland Security inspector general cleared him of any wrongdoing, according to Yahoo News . Now in private practice, Bakaj and Zaid, who work together on whistleblower and security clearance cases, are representing the whistleblower

This story has been updated.