Mr. Trump directed Mick Mulvaney, the acting White House chief of staff, to pause the aid in the days before his July phone call with Mr. Zelensky, senior administration officials said. The Trump administration abruptly unfroze the package this month amid mounting questions about what Mr. Trump was doing with Ukraine.

Why is this coming up now?

An intelligence official filed a whistle-blower complaint last month about the president’s actions. The inspector general for the intelligence community deemed the complaint “credible” and “urgent” and forwarded it to the acting director of national intelligence, Joseph Maguire, under a law that says such complaints must be shown to Congress within a week.

But Mr. Maguire refused to share the complaint with Congress, saying the Justice Department disagreed with the inspector general’s conclusion that its subject matter was covered under the law that requires disclosing such complaints to Congress. The issue came into the open when the House Intelligence Committee chairman, Representative Adam B. Schiff, Democrat of California, sent an angry letter to Mr. Maguire on Sept. 10, accusing him of violating the law.

Mr. Maguire relented as the impeachment inquiry came into focus. He was expected to release a redacted version of the complaint in coming days, people familiar with the situation said late on Tuesday. Administration officials were also said to be working on a deal to allow the whistle-blower to file the complaint to congressional investigators.

Mr. Maguire is set to testify about the matter on Thursday.

What did the whistle-blower claim?

The complaint’s full details remain a mystery, as does the whistle-blower’s identity. Because Mr. Trump and Mr. Giuliani have openly acknowledged pressuring Ukraine about “corruption” and the Bidens, it is not clear how much the complaint involves that is not already in plain view.

But in a closed-door briefing with the House Intelligence Committee, the inspector general for the intelligence community, Michael Atkinson, said that the complaint involved multiple actions, according to two officials familiar with his briefing, suggesting that the matter goes beyond Mr. Trump’s July phone call.

What does Mr. Trump say?

The president insists that he has been unfairly accused, saying — without offering evidence — that the whistle-blower is “partisan” and that Democrats and the news media are initiating a new “witch hunt” against him. Mr. Trump has also said that he is aware that his conversations with foreign leaders are monitored by numerous government officials and that he would not incriminate himself so easily.