Michael Atkinson

Intelligence community inspector general who first notified Congress of the whistle-blower complaint, met privately with the House Intelligence Committee on Friday to discuss how he had determined that the allegations in the complaint were credible.

Kurt D. Volker

The former U.S. special envoy to Ukraine, testified before congressional investigators and provided text messages revealing that Mr. Trump’s envoys made clear to the Ukrainian president that he would need to agree to Mr. Trump’s requests for investigations before visiting the Oval Office.

Bill Taylor

Career diplomat and a former United States ambassador to Ukraine. Taylor has served as the chargé d’affaires for Ukraine since June 2019.

Steven A. Linick

State Department’s independent watchdog, provided senior congressional staff members with packets of documents that sent rumours flying on Capitol Hill but did not appear to include any explosive revelations.

Joseph Maguire

Acting director of national intelligence, shared the whistle-blower’s complaint with Congress weeks after he was mandated to provide it. In testimony the next day, he defended the whistle-blower’s actions, as well as his own handling of the case.

Secretary of State Mike Pompeo 

Was subpoenaed to hand over documents by Friday, Oct. 4, after initial requests were ignored. After Mr. Pompeo revealed that he had been on the call between Mr. Trump and the Ukrainian president, congressional investigators said that he should not be involved in decisions about what documents should be given to the House.

Rudolph W. Giuliani

The president’s personal lawyer, Trump asked Mr. Guiliani to work with Ukrainian officials to seek damaging information about Mr. Biden and his family.

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Vice President Mike Pence

Was asked to hand over documents that could reveal “any role” he had played in Mr. Trump’s pressure campaign on Ukraine.

The Whistleblower 

C.I.A. officer who was detailed to work at the White House.

Marie L.Yovanovitch

Former U.S. ambassador to Ukraine, was recalled from her post months ahead of schedule.

George Kent 

Deputy assistant secretary of state for European and Eurasian affairs overseeing American policy toward Ukraine.

T. Ulrich Brechbuhl

State Department counselor, listened in on the call between Mr. Trump and the Ukrainian president, according to the whistle-blower complaint.

Gordon D. Sondland

U.S. ambassador (political appointee) to the European Union. Along with Mr. Volker advised Ukrainian leadership about how to “navigate” Mr. Trump’s demands, according to the complaint.

Lev Parnas

An energy company executive, has known Mr. Giuliani for years and helped connect Mr. Giuliani and Ukrainian prosecutors.

Igor Fruman

Another energy company executive, worked with Mr. Parnas to gather information in Kiev about the Bidens and others in Ukraine.

Semyen Kislin

Another associate of Mr. Giuliani.