That aide, Fiona Hill, testified privately to House impeachment investigators on Monday.

Three people who heard her testimony told The Times that Ms. Hill had quoted Mr. Bolton from another conversation: “Giuliani’s a hand grenade who’s going to blow everybody up.”

Background: Ms. Hill was the first former White House official to testify in the impeachment inquiry. She left her post before the July phone call in which Mr. Trump asked Ukraine’s president to investigate Democrats.

What’s next: Investigators are scheduled today to interview George Kent, a State Department official and Ukraine expert. This week is also the deadline for responses to document requests from figures including Mr. Giuliani, Vice President Mike Pence and the acting White House chief of staff, Mick Mulvaney. The White House has vowed not to cooperate.

The Daily: Today’s episode is about the former U.S. ambassador to Ukraine.