Democrats’ argument for impeachment is straightforward: President Donald Trump ran a shadow foreign policy campaign to pressure Ukraine to launch investigations that would help him politically. To make their case, House investigators have called on participants in the effort, including career diplomats and others who can provide firsthand accounts. A look at the central figures:

The whistleblower

The whistleblower, whose identity hasn’t been revealed, works in the intelligence community, currently outside the White House. The whistleblower filed a complaint Aug. 12 saying he had “received information from multiple U.S. government officials that the President of the United States is using the power of his office to solicit interference from a foreign country in the 2020 U.S. election.” The whistleblower said he was “not a direct witness” to most of the events but had access to top officials who were. Despite laws protecting whistleblowers, Trump has called for the whistleblower to be unmasked.

Rudy Giuliani

Giuliani, Trump’s personal lawyer, pursued a back-channel lobbying effort to persuade Ukrainian prosecutors to reopen an investigation into a Ukrainian energy company, Burisma Holdings, which had former Vice President Joe Biden’s son on its board. Trump held up military aid to Ukraine and dangled a White House visit, allegedly in exchange for opening the investigation. Two of Giuliani’s business associates, Lev Parnas and Igor Fruman, have been charged with funneling hundreds of thousands of dollars to U.S. officials and a political action committee that backed Trump. They have pleaded not guilty.

Kurt Volker

A career U.S. diplomat who served under former Presidents Ronald Reagan and George W. Bush, Volker has been Trump’s special envoy to Ukraine, a volunteer position in which he also allegedly played a role in the shadow campaign. He abruptly resigned in late September after the whistleblower complaint surfaced. According to the whistleblower, Volker visited the Ukrainian capital the day after Trump’s July 25 phone call with Ukranian President Volodymyr Zelensky, providing advice about how to “navigate” Trump’s request for an investigation. Volker was working in concert with Giuliani.

Gordon Sondland

Sondland is a Trump inauguration donor who was later tapped as his envoy to the European Union, where he allegedly played a central role in the Ukraine shadow campaign. The whistleblower complaint identifies Sondland and Volker as a team that bridged the gap between the State Department and Giuliani. Witnesses in the impeachment investigation have testified that Sondland said Trump made a White House meeting with Zelensky and the delivery of $391 million in military aid for Ukraine contingent on the investigations he sought into former Vice President Joe Biden and his son Hunter.

Mick Mulvaney

Mulvaney currently serves as director of the Office of Management and Budget and as acting White House chief of staff. Mulvaney’s dual roles put him in a central position in the Ukraine scandal because the OMB was the agency that delayed the military aid to Ukraine. In a White House news conference on Oct. 17, he undercut the White House argument that there was “no quid pro quo” when he said that the aid was tied to Trump’s demand for an investigation into the 2016 election. “Get over it,” he said. “There’s going to be political influence in foreign policy.” Mulvaney later tried to walk back his remarks.

Marie Yovanovitch

The career diplomat served as U.S. ambassador to Ukraine from 2016 until 2019, when she was recalled by Trump after what she described was a “concerted campaign” by Giuliani and others against her. Yovanovitch was viewed with suspicion by Giuliani and Trump. After Trump asked for an investigation, Zelensky asked Trump for information on Yovanovitch. Trump called her “a bad ambassador” loyal to former President Barack Obama and said “she’s going to go through some things.” She testified that her departure came as a direct result of pressure from Trump on the State Department.

William Taylor

Taylor is the top U.S. envoy to Ukraine. A career diplomat, he took over as acting ambassador after Yovanovitch left. He testified that Sondland told him Trump had made aid to Ukraine entirely dependent on a public promise to open investigations because he wanted Zelensky “in a public box.” Taylor’s texts to Sondland have been publicly released: “Are we now saying that security assistance and WH meeting are conditioned on investigations?” and “I think it’s crazy to withhold security assistance for help with a political campaign.” Sondland responded by telling Taylor not to text about the matter anymore.

Alexander Vindman

Vindman is a U.S. Army lieutenant colonel who handles European affairs for the National Security Council. He told investigators the partial transcript the White House released of Trump’s phone call with Zelensky omitted key words and phrases that he had attempted to have restored. But he said that two omissions weren’t fixed. Vindman also testified that he raised objections with the top National Security Council lawyer twice about Trump pressuring Ukraine to investigate a political opponent, saying he “did not think it was proper to demand that a foreign government investigate a U.S. citizen.”

George Kent

The career diplomat was the deputy assistant secretary of state responsible for Ukraine. Kent reportedly testified that he was told to “lay low” on Ukraine policy and let Energy Secretary Rick Perry, Sondland and Volker handle it. Kent also told lawmakers that Giuliani and a Ukrainian prosecutor spread disinformation to undermine Yovanovitch and clear a roadblock to the agenda Giuliani and his clients were pursuing there. Trump “wanted nothing less than President Zelensky to go to a microphone and say, ‘Investigations, Biden and Clinton,’ ” Kent told House impeachment investigators.

Fiona Hill

Hill served as the senior official for Russia and Europe on the National Security Council, reporting to then-national security adviser John Bolton. She left the NSC this summer. She told House investigators she confronted Sondland about Giuliani’s work in Ukraine, which she said wasn’t coordinated with U.S. officials responsible for the region. She also said Bolton was angry about Giuliani’s activity, referring to it as a “hand grenade” that’s going to “blow everybody up” and a “drug deal” that Sondland and Mulvaney were “cooking up.” Hill said Bolton instructed her to raise concerns with White House lawyers.

Mike Pence

The vice president was directly involved in Ukraine discussions at times. He was slated to attend Zelensky’s inauguration in May, but Trump reportedly instructed him not to go, sending Perry instead. On Sept. 1, Pence met with Zelensky in Warsaw, telling him that the aid wouldn’t be distributed to Ukraine until it did more to fight corruption, which House Democrats argue would have been understood as investigating the Bidens.

William Barr

It’s unclear how involved the attorney general is in the Ukraine investigation, but in his phone call with Zelensky, Trump twice brought up Giuliani and Barr in the same breath: “I will have Mr. Giuliani give you a call and I am also going to have Attorney General Barr call and we will get to the bottom of it,” he told the Ukrainian president. “I will tell Rudy and Attorney General Barr to call.” Recently, the Justice Department, which Barr oversees, has sought some distance from the White House and Giuliani, particularly on matters relating to Trump’s dealings on Ukraine.

Rick Perry

The Energy secretary was one of the “three amigos” charged with running Trump’s policy on Ukraine, along with Sondland and Volker, lawmakers were told by Kent, the State Department official.