Donald Trump’s increasingly brazen attempts to pressure Volodymyr Zelensky into opening an investigation into Joe Biden have put a number of his cronies under the microscope. Rudy Giuliani, his personal lawyer, may have the most to lose, as prosecutors in New York scrutinize his work in Ukraine. Energy Secretary Rick Perry also appears in an unflattering light, as do diplomats Kurt Volker and Gordon Sondland, all of whom were apparently involved in the Trump-Giuliani scheme. More recently, the spotlight has turned to “Acting” Chief of Staff Mick Mulvaney, who is described in a new report as a lynchpin of Trump’s campaign to squeeze Zelensky.

According to the Washington Post, it was Mulvaney who coordinated Perry, Volker, and Sondland’s efforts to abscond with the Ukraine portfolio, moving the administration’s dealings with Kiev out of traditional foreign policy channels and onto more corrupt footing. Giuliani told the Post that he could not recall “any substantive conversation with Mick,” nor could he remember Mulvaney “approving, disapproving, getting involved, [or] having an interest” in the Ukraine imbroglio. But current and former United States officials have placed the acting chief of staff at the center of the scandal in Capitol Hill testimonies in recent days.

Fiona Hill, who had been Trump’s top Russia adviser, dropped Mulvaney’s name in her bombshell testimony on Monday. Speaking to House lawmakers, who are conducting an impeachment inquiry into the president, Hill reportedly testified that then-national security adviser John Bolton had raised alarms about the pressure campaign on Ukraine, describing Giuliani as a “hand grenade” and instructing her to tell White House lawyers that he was “not part of whatever drug deal Sondland and Mulvaney are cooking up.” Hill also told congressional investigators that Sondland had described an “agreement” with Mulvaney to broker a White House meeting for Zelensky, who took office in May, if he authorized probes into the Bidens and the origins of the Russia inquiry.

George Kent, the deputy assistant secretary of state responsible for Ukraine, went even further in his testimony Tuesday, telling lawmakers that Mulvaney had told him to “lay low” and focus on other countries in his portfolio—giving control of the administration’s Ukraine policy to the so-called “three amigos” in what the top State department official took as a troubling sign that the White House was pursuing its own political agenda. Mulvaney told Kent to defer to Perry, Volker, and Sondland on matters related to Ukraine, Kent said in his testimony, and felt he was being sidelined “because what he was saying was not welcome” in the administration.

Perry, Sondland, and Volker have all come under intense scrutiny in the Ukraine scandal; explosive text messages released earlier this month show Volker, then-ambassador to Kiev, and Sondland, envoy to the European Union, suggesting that a White House visit for Zelensky and security aid for Ukraine was contingent on the probes Trump and Giuliani were seeking. Bill Taylor, Charge d'Affaires at the U.S. embassy in Ukraine, raised concerns about the apparent quid pro quo in exchanges with the other officials: “I think it’s crazy to withhold security for help with a political campaign,” he wrote to Sondland at one point. Volker abruptly resigned his post earlier this month, and Perry’s involvement in the scandal has threatened his job security as Energy secretary.

Could Mulvaney’s position as acting chief of staff become similarly tenuous? Already, the former congressman is facing questions about his misadventures with the “amigos”—Sondland, Volker, and Perry. He is also expected to be called before Congress to testify about the hold he placed on some $400 million in military aid for Ukraine, on Trump’s orders, days before the president’s July 25 phone call with Zelensky.

The White House has already begun an internal review of the matter, reportedly encouraged by Mulvaney, that some insiders fear could be an effort to find a fall guy. But it’s also possible that Mulvaney, in his effort to please the president, could become a scapegoat himself.