Kurt Volker, former part-time special envoy to Ukraine, and Gordon Sondland, ambassador to the European Union. Both men consulted with Mr. Giuliani about his Ukraine project. On July 26, one day after Mr. Trump’s call with Mr. Zelensky, Mr. Volker and Mr. Sondland met with Ukrainian officials and reportedly offered advice on how to “navigate” Mr. Trump’s requests. Did they, as the whistle-blower claims, at some point become concerned about Mr. Giuliani’s work and seek to “contain the damage”? Mr. Volker resigned his post on Friday. Why?

Mike Pence, vice president. In his conversations with Ukrainian officials, including his Sept. 1 meeting with Mr. Zelensky, was there any mention of Mr. Biden or of the delayed military funding package? When asked at a news conference on Sept. 2 if he could assure Ukraine that the two issues were not linked, Mr. Pence ducked the question. Mr. Pence should also explain why Mr. Trump directed him to cancel his plans to attend Mr. Zelensky’s inauguration in May.

Mr. Trump himself has suggested looking into Mr. Pence’s interactions with Ukrainian officials. “And I think you should ask for VP Pence’s conversation, because he had a couple conversations also,” he told reporters on Wednesday.

John Bolton, former national security adviser. Mr. Bolton was forced out of the White House in September. What did he know about Mr. Trump’s pressure campaign? Mr. Bolton is said to have pushed for the withheld military aid to be released. What explanation did he receive for it being withheld?

Michael Atkinson, the inspector general of the intelligence community. Mr. Atkinson reviewed the whistle-blower complaint, deemed it both “urgent” and “credible,” and forwarded it to Joseph Maguire, the acting director of national intelligence. After Mr. Maguire declined to pass the complaint along to Congress, as indicated by federal law, Mr. Atkinson chose to alert lawmakers to its existence himself. What explanation did Mr. Maguire give for not forwarding the complaint? How did he respond when Mr. Atkinson informed him that he would be alerting Congress?

Lawmakers will also need to hear from whomever was charged with moving the transcript of Mr. Trump’s July 25 call from the usual computer system to the special server, maintained by the National Security Council, reserved for “classified information of an especially sensitive nature.” Who directed this action? (On Friday, a White House official told CNN that N.S.C. attorneys did so.) Who else knew about it? Did anyone object at the time? Have other such conversations been improperly stashed in the system, as the whistle-blower alleged? (It has been reported that reconstructed transcripts of phone calls Mr. Trump had with the Saudi royal family and with Vladimir Putin, the Russian president, were stored on the server as well.) It’s worth remembering that one of the biggest bombshells of the Watergate hearings came from Alexander Butterfield, a relatively obscure administration staffer, who shared what he knew about the White House taping system.

Then there are the “multiple U.S. government officials” whom the whistle-blower cites as his sources — the ones whom Mr. Trump has compared to spies and has implied deserve to be executed for treason.