The whistleblower complaint that helped spark the impeachment inquiry into President Donald Trump has a dizzying cast of characters.

Here’s a guide to the major players in the forthcoming investigation:

The whistleblower

The identity of the whistleblower who rang the alarm on Trump’s call with the president of Ukraine is the biggest question in Washington — even for Trump and lawmakers. The New York Times reported it is an intelligence official, not part of the White House staff, who sent a complaint to the Inspector General of the Intelligence community. The complaint contains accounts and concerns reported to the whistleblower. Lawmakers have said this person may testify soon, which would make it hard to keep his or her identity secret for long.

Volodymyr Zelensky, Ukraine’s president

Zelenksky is a comedian-turned-politician who was elected to be Ukraine’s president in April. On July 25, 2019, Trump and Zelensky had a phone call during which Trump said he needed a “favor” and asked him to look into former Vice President Joe Biden, his son and their involvement in Ukraine. The request came a week after Trump ordered a hold on nearly $400 million in U.S. aid to Ukraine.

Rudolph Giuliani, Trump’s personal lawyer

The name of the president’s personal lawyer is all over the whistleblower’s complaint. According to White House notes on the president’s July phone call with Zelensky, Trump asked him to meet or speak with Giuliani. According to the complaint, Giuliani met with Zelensky’s adviser Andriy Yermak in Madrid in early August and had met with former Ukrainian prosecutor general Yuri Lutsenko earlier this year. He had a skype call with former Prosecutor General Shokin in 2018. What is unclear is what role the president’s personal lawyer, who does not have a foreign policy or White House job, would have in these discussions.

Rep. Nancy Pelosi, House speaker

Pelosi had major reservations about launching an impeachment inquiry, but mounting pressure made her and other hesitant Democrats change course. On Sept. 24, Pelosi directed the House committees to continue their investigations into the Ukraine matter and called it a formal impeachment inquiry.

Rep. Adam Schiff, chairman of the House Intelligence Committee

Schiff’s committee is charged with leading the Ukraine investigation. The committee also investigated Russian interference in the 2016 presidential election, but ended the probe earlier this year.

Joe Biden, former vice president & Hunter Biden

Hunter Biden joined the board of Ukraine’s Burisma Holdings, Ukraine’s largest private gas company, in 2014 while Joe Biden was vice president. The company faced allegations of corruption due to the owner’s close ties to Ukraine’s ousted president. Joe Biden was one of a number of western leaders calling on Ukraine to fight corruption. Trump has accused the former vice president of acting to protect his son.

William Barr, U.S. attorney general

The whistleblower complaint says Barr “appears to be involved” in the controversy as well. During Trump’s phone call with Zelensky, according to notes released by the White House, he offered to have Barr call Zelensky to “get to the bottom” of his suspicions about the Bidens. Though Barr is named in the complaint, he does not appear to be recusing himself from considering its merits.

Marie Yovanovitch, former U.S. ambassador to Ukraine

Yovanovitch became the U.S. ambassador to Ukraine while Zelensky’s predecessor was in office. She became a vocal critic of Ukrainian officials, calling on them to fight corruption. Yuri Lutsenko, the country’s former prosecutor general, responded to Yovanovitch’s pressure by accusing her of obstructing Ukrainian law enforcement in corruption cases, according to the whistleblower complaint. In May, Yovanovitch was suddenly recalled from her post. She told House lawmakers during an Oct. 11 testimony that the Deputy Secretary of State informed her of a “concerted campaign” against her and pressure from the president to remove her.

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

A day after the Trump-Zelensky phone call, Volker met with the Ukrainian president to give “advice to the Ukrainian leadership about how to ‘navigate’ the demands that the president had made,” according to the whistleblower’s complaint. The State Department said Volker also arranged a meeting between Giuliani and Yermak. Volker gave hours of closed-door testimony before members of three House committees on Oct. 3. Volker provided text messages that revealed more details about the Trump administration’s contact with Ukraine.

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

Though his official duties do not involve U.S.-Ukraine relations, text messages show Sondland communicated regularly with Volker about the White House effort to pressure Ukraine to investigate Biden and his son. Under subpoena, Sondland gave closed-door testimony before three House committees on Oct. 17, despite efforts from the Trump administration to block him from appearing.

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

Taylor is a West Point graduate who was named Ambassador to Ukraine by President George W. Bush. He was asked to return to diplomacy after Yovanovitch was ousted by Trump in May.

In text messages released by Volker, Taylor expressed concern about Trump’s intentions with Ukraine, at one point saying, “As I said on the phone, I think it’s crazy to withhold security assistance for help with a political campaign.”

On Oct. 22, Taylor told House lawmakers he believed the Trump administration had withheld military aid to Ukraine in exchange for an investigation into political rivals. His opening statement detailed weeks of conversations concerning the use of military aid to pressure Zelensky to investigate a company where former Vice President Joe Biden’s son served on the board.

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

Army Lt. Col. Vindman was one of the first officials (along with Jennifer Williams) who listened in on the July 25 phone call between Trump and Zelensky to testify in the impeachment inquiry. In a six-page opening statement released to House lawmakers, Vindman said he twice raised concerns about an earlier July meeting where he said Amb. Gordon Sondland “emphasized the importance that Ukraine deliver the investigations into the 2016 election, the Bidens, and Burisma.”– the first concern was directly to Sondland and the second was to the NSC’s lead counsel. Vindman also recounted his thoughts on the July 25 Trump-Zelensky call: “I did not think it was proper to demand that a foreign government investigate a U.S. citizen, and I was worried about the implications for the U.S. government’s support of Ukraine.”

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

McKinley told House lawmakers on Oct. 16, that he resigned from his post the prior week as a result of: “the failure, in my view, of the State Department to offer support to Foreign Service employees caught up in the impeachment inquiry; and, second, by what appears to be the utilization of our ambassadors overseas to advance domestic political objectives.” McKinley served as a diplomat for more than 30 years, and resigned from his post in October.

During his testimony, he also expressed disapproval of the treatment of former U.S. Ambassador Marie Yovanovitch.

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

As a senior State Department official, Kent oversees policy for Ukraine and several other nations. Kent told House impeachment investigators on Oct. 15 that he was sidelined from handling policy decisions around Ukraine after a May meeting about Ukraine in the Oval Office between President Donald Trump, European Union Ambassador Gordon Sondland, and other officials. Kent said after the meeting he was asked to “lay low” on policy issues regarding Ukraine.

Rick Perry, U.S. energy secretary

In early October, Trump stated that he only made the widely discussed July 25 phone call to Zelensky at Perry’s request, according to Axios. Perry has acknowledged that he asked Trump to call the Ukrainian president “multiple times,” but has said he wanted the focus to be on energy issues. He added that Joe and Hunter Biden were not the source of his interest in Ukraine. Perry has refused to comply with a congressional subpoena requesting documents related to his work in Ukraine. On Oct. 17, Perry said that he plans to leave his post by the end of the year.

Tim Morrison, former senior director for Europe and Russia on the National Security Council

During his closed-door testimony to House lawmakers on Oct. 31, Morrison confirmed he was on the July 25 phone call between Trump and Zelensky, and shared his concerns that if the call transcript leaked, it could result in negative consequences for the Trump administration. He added, however, “I want to be clear: I was not concerned that anything illegal was discussed.” He also corroborated parts of William Taylor’s testimony on Oct. 22. Morrison resigned on the eve of his closed-door testimony, though he said it is unrelated to the inquiry.

Fiona Hill, former Russia adviser for President Trump

During her meeting with lawmakers on Oct. 14, Hill shared the concerns that then-National Security Adviser John Bolton expressed about the administration’s efforts to pressure Ukrainian officials. She told lawmakers about a July meeting during which Ambassador Sondland discussed reopening some “investigations.” Hill and others reportedly understood this to be referencing the Bidens. Hill later met with a National Security Council lawyer to relay her misgivings about the situation and Giuliani’s involvement with Ukraine.

Laura Cooper, deputy assistant secretary of defense

Cooper’s work focuses on Russia, Ukraine, and Eurasia. On Oct. 23, Cooper provided more details to lawmakers about the hold on Ukraine military aid. According to Cooper, she heard that Ukrainians had become concerned about the hold on U.S. aid in August. She spoke about a conversation with Kurt Volker during which she says he told her about an effort to see if the Ukrainians “would somehow disavow any interference in U.S. elections and would commit to the prosecution of any individuals involved in election interference.” She indicated that these actions might result in a lift on the hold.

Jennifer Williams, special adviser to the vice president for Europe and Russia

Jennifer Williams is a foreign service officer focused on Europe and Russia who is detailed to Vice President Pence’s staff. She’s the first member of the Vice President’s staff to participate in the impeachment inquiry. Williams was instructed by the White House to not testify, but she complied with her subpoena and appeared behind closed doors for the committee on November 7.

David Hale, the undersecretary of state for political affairs

Hale, the third-ranking official in the State Department, testified in a closed-door meeting on Nov. 6. He was expected to provide more information about the circumstances of Amb. Yovanovitch’s sudden removal from her post in Ukraine. According to Yovanovitch’s closed-door testimony, she asked the State Department to issue a statement defending her, which they did not do.

David Holmes, political counselor for the U.S. Embassy in Ukraine

During his closed-door hearing with lawmakers on Nov. 15, Holmes stated that while at a restaurant in Ukraine he overheard a phone call between Amb. Sondland and Trump. Holmes said he could hear the president ask about “investigations” that he wanted Ukraine to open. That call took place one day after the one on July 25 between Trump and Zelensky.

From Holmes private testimony: “I then heard President Trump ask, quote, ‘So he’s going to do the investigation?’ unquote. Ambassador Sondland replied that, “He’s going to do it,” adding that President Zelensky will quote, ‘Do anything you ask him to.’”

Senate Republicans

Even if the House votes to impeach Trump, the Senate would be the body to ultimately remove him from office. Senate Majority Leader Mitch McConnell has said the Senate will have no choice but to hold a trial if the House votes to forward articles of impeachment. But nothing yet suggests that Republicans will abandon Trump, so removal is unlikely.

The unknowns

The whistleblower complaint says there are a number of officials who were concerned about Trump’s call with Zelensky, any of whom could be asked to testify. Also likely to be questioned include individuals listening to the July 25 phone call. The whistleblower also claimed that the notes from the call were moved into an especially secure system generally reserved for national security secrets, and lawmakers are sure to want to speak to whoever was involved with that decision.

Other experts called to tesify:

Noah Feldman, professor at Harvard Law School

Feldman specializes in constitutional studies, especially the law and religion, free speech, constitutional design, and the history of legal theory. He also served as senior constitutional adviser to the Coalition Provisional Authority in Iraq.

Pamela Karlan, professor at Stanford Law School

Karlan has served as a commissioner on the California Fair Political Practices Commission, an assistant counsel and cooperating attorney for the NAACP Legal Defense Fund, and a Deputy Assistant Attorney General in the Civil Rights Division of the U.S. Department of Justice.

Michael Gerhardt, professor at University of North Carolina Law School

Gerhardt testified before Congress as a joint witness during the impeachment process for former President Bill Clinton. He is also a former media director for Al Gore’s presidential campaign.

Jonathan Turley, professor at The George Washington University Law School

Turley focuses on public interest law, and has written a number of columns critiquing the impeachment inquiry. Turley also testified as a constitutional expert during the Clinton impeachment. His testimony was requested by the Republicans.

Barry Berke, attorney

Berke is a legal consultant to the Democrats on the Judiciary Committee. In that role, Berke questioned Corey Lewandowski during his House Judiciary hearing in September. Berke is currently on leave from the New York law firm, Kramer Levin, in order to work with the House committee.

Stephen Castor, attorney

Castor is the general counsel for the House Oversight and Reform Committee, representing the Republicans. Castor previously played a central role in the investigation of the George W. Bush administration after Hurricane Katrina, and questioned witnesses during the Benghazi hearings in 2014. Castor has worked on the Committee for more than 14 years.

Daniel Goldman, attorney

A former assistant U.S. attorney in Manhattan, Daniel Goldman helped prosecute many high profile white collar cases before taking a position at NBC as an on-air legal analyst. Goldman is the Democrats’ lead attorney in the impeachment hearings.