Former national security adviser John Bolton was so concerned about Rudy Giuliani’s efforts to pressure Ukraine for political help that he ordered an aide to warn White House attorneys about his behavior.

According to a seriesof reports on Monday evening, Fiona Hill, the Trump administration’s former top Russia adviser, was instructed to notify the chief lawyer for the National Security Council that Giuliani, Trump’s personal attorney, was working on a shadow foreign policy effort outside the bounds of the State Department. Hill detailed the internal concerns about Giuliani’s efforts during 10 hours of closed-door testimony as part of House Democrats’ impeachment inquiry.

Hill first raised the concerns to the National Security Council on July 10 following a policy fight in the White House between Bolton and Gordon Sondland, Trump’s ambassador to the European Union, The New York Times reported Monday. Sondland was reportedly working with Giuliani to pressure Ukrainian leadership to investigate one of the president’s prime political rivals, former Vice President Joe Biden, and his son Hunter Biden.

Text messages released earlier this month by House Democrats appear to show Sondland was deeply enmeshed in the effort to persuade Ukrainian President Volodymyr Zelensky to open up an investigation into the Bidens, neither of whom has been formally accused of any wrongdoing. Sondland is expected to speak with lawmakers later this week about his work.

Bolton also told Hill to alert White House attorneys that Giuliani was working with White House chief of staff Mick Mulvaney on the rogue policy. It was not the first time Bolton, who was fired last month, expressed concern about Trump’s lawyer.

“Giuliani’s a hand grenade who’s going to blow everybody up,” Bolton said during an earlier conversation related to concerns over Giuliani’s work, according to Hill’s testimony to lawmakers, the Times reported.

Giuliani told The Washington Post later Monday that he didn’t know Hill and “can’t figure out what she is talking about.”

“I reported everything back to [the State Department],” he told the Times, echoing his assertions that his Ukraine work came at the direction of the State Department. “Nothing shadowy about it.”

Hill, who left the White House on July 19, also told lawmakers she was not part of Trump’s call that set off the impeachment inquiry, although she said she was opposed to the discussion. During the conversation, which took place on July 25, just days after Trump ordered nearly $400 million in military aid to Ukraine be put on hold, the president repeatedly pressed Zelensky to help investigate the Bidens.

Trump has denied he demanded any quid pro quo for America’s military aid, but critics have called the effort an unprecedented move to pressure a foreign government to perform a political favor.

Giuliani has emerged as a central figure of the impeachment inquiry and is reportedly under investigation to determine if he broke any foreign lobbying laws. According to multiple reports, the former New York City mayor was also instrumental in the removal of Marie Yavanovitch, the respected U.S. ambassador to Ukraine. CNN reported Monday that Hill referred to that effort as part of Giuliani’s “shadow foreign policy.”

Hill was the first former White House official to testify as part of the impeachment inquiry. The Trump administration has been attempting to stonewall lawmakers’ efforts to investigate the Ukraine call, and the White House moved to limit Hill’s testimony on Monday, although it didn’t try to stop her from speaking to investigators, according to Axios.

House lawmakers issued a last-minute subpoena on Monday to force her to speak.