Then-national security adviser John Bolton was so disturbed by the efforts to get the Ukrainians to investigate President Donald Trump’s political opponents that he called it a “drug deal,” former White House official Fiona Hill reportedly told Congress on Monday.

Hill, the former top Europe expert in Trump’s White House, testified that Bolton told her over the summer that he wanted no part of the effort, which he said involved acting White House chief of staff Mick Mulvaney, a person in the room for Hill’s testimony told NBC News.

Bolton also was said to have referred to Rudy Giuliani as a "hand grenade."

Giuliani has acknowledged in recent interviews that he had asked a Ukrainian prosecutor for information about former Vice President Joe Biden.

The New York Times first reported details of Hill's testimony Monday night.

Trump fired Bolton in September.

Hill testified that Bolton told her to report the situation to the top lawyer at the National Security Council, John Eisenberg, according to the person in the room for Hill’s testimony during the closed-door hearing.

National security adviser John Bolton with President Trump in the Oval Office on Sept. 28, 2018. Oliver Contreras / The Washington Post via Getty Images file

Hill told lawmakers she considered what was happening to be a clear counterintelligence risk to the United States, the person said.

Giuliani said that he was disappointed to learn about Bolton's reported comments.

"I always liked and respected John," the president's personal attorney said after hearing about the testimony. "I’m very disappointed that his bitterness drives him to attack a friend falsely and in a very personal way. It’s really ironic that John Bolton is calling anyone else a hand grenade. When John is described by many as an atomic bomb."

Requests for comment to the White House and Bolton were not immediately returned Monday night.

Also Monday, correspondence between the White House deputy counsel and Hill’s lawyers show the White House tried to limit what Hill could say to Congress in its impeachment inquiry.

The letters, obtained by NBC News, illustrate that while the White House did not try to block Hill from testifying, it did tell Hill’s lawyers about four areas that could potentially fall under executive privilege.

Hill testified under subpoena Monday to the House. Previously, she agreed to testify at Congress' written request.

The House has launched an impeachment inquiry over matters that include Trump’s communication with Ukrainian President Volodymyr Zelenskiy that involved a request for an investigation by that country into Biden and his son.

There is no evidence of wrongdoing by either Joe or Hunter Biden.

CLARIFICATION (Oct. 15, 2019, 1:35 p.m.): An earlier version of this article, citing The New York Times, said Fiona Hill testified that then-national security adviser John R. Bolton said he was not part of "whatever drug deal" Rudy Giuliani and acting White House chief of staff Mick Mulvaney were "cooking up.” The Times later said that while one person in the room said Bolton had referred to Giuliani and Mulvaney, two others said he cited Mulvaney and Gordon Sondland, the American ambassador to the E.U., not Giuliani.