(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 turned Democrats toward an impeachment investigation. There is no evidence that either Biden did anything wrong.

       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

       The former Kansas congressman and CIA chief has emerged as one of Trump's most trusted advisers. While Pompeo is not mentioned by name in the whistleblower complaint, Giuliani has said the State Department helped set up his meetings with Zelensky aides. Pompeo was subpoenaed by House committees as part of the impeachment inquiry on September 27.

       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.

       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.

       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.

       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.

       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.

       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

       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 Yerkmak, Zelensky's aide. He is essentially a volunteer, however, and still works for BGR group, a Washington lobbying firm that has represented the government of Ukraine. Volker reportedly resigned his position on Friday, three sources familiar with the matter told CNN.

       Gordon Sondland, United States Ambassador to the EU

       Mentioned in the complaint alongside Volker, the two were said by the whistleblower to be advising the Ukrainian leadership about how to deal with Trump and Giuliani.

       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."

       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.

       More coverage of the whistleblower complaint

       The whistleblower complaint, annotated

       The Point: The 1 piece of the whistleblower complaint that poses the biggest threat to Trump

       Whistleblower timeline: Team Trump contacts and Ukraine

       Here's what Republican senators are saying about the whistleblower complaint

       More than half the House of Representatives support impeachment inquiry

       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 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.

       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

       The former Kansas congressman and CIA chief has emerged as one of Trump's most trusted advisers. While Pompeo is not mentioned by name in the whistleblower complaint, Giuliani has said the State Department helped set up his meetings with Zelensky aides. Pompeo was subpoenaed by House committees as part of the impeachment inquiry on September 27.

       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.

       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.

       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.

       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.

       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.

       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

       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 Yerkmak, Zelensky's aide. He is essentially a volunteer, however, and still works for BGR group, a Washington lobbying firm that has represented the government of Ukraine. Volker reportedly resigned his position on Friday, three sources familiar with the matter told CNN.

       Gordon Sondland, United States Ambassador to the EU

       Mentioned in the complaint alongside Volker, the two were said by the whistleblower to be advising the Ukrainian leadership about how to deal with Trump and Giuliani.

       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."

       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 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 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.

       CNN's Allison Malloy, Will Mullery, Daniel Dale and Veronica Stracqualursi contributed to this report.