Republicans may be wary of another benefit to this kind of private digging: It allows Democrats to more carefully control the case they are building, avoiding potentially embarrassing public testimony from witnesses that may undercut or deflate elements of the story they hope to tell.

Is there precedent for a closed-door impeachment inquiry?

Keeping the first phase behind closed doors is not just investigative best practice, Democrats assert, but the way federal investigations, whether in Congress or the executive branch, typically take place.

Republicans have frequently noted that the impeachment proceedings against Presidents Richard M. Nixon and Bill Clinton were open to the public, but in those cases, there had already been extensive federal investigations that provided the backbone for the House’s work. (In the Nixon case, House investigators also did significant investigative work in private before making their public case.)

Confidential interviews have also driven many other high-profile investigations, such as the one opened by Republicans during the Obama administration into the attack on American diplomatic facilities in Benghazi, Libya, in 2012. The Republican leading that inquiry mounted a vigorous defense of his private hearings at the time.

“I can get more information in a five-hour deposition than I can in five minutes of listening to a colleague ask questions in committee hearings,” Trey Gowdy, the former Republican congressman from South Carolina who led the inquiry, told ABC in 2014. “If it’s about getting the information, then you want to use the investigatory tool that is most calculated and gets you the most amount of information and that’s not five minutes in a committee hearing.”

Republicans retort that though the Benghazi investigation was serious and implicated Hillary Clinton, the former secretary of State and Democratic presidential nominee, it was not as weighty as an impeachment inquiry.

Will Democrats ever take their inquiry public?

Democrats insist they will eventually share what they find and convene public hearings to present witness testimony. The question is when.