WASHINGTON (AP) — National security adviser John Bolton was so alarmed by Rudy Giuliani's back-channel activities in Ukraine that he described President Donald Trump's personal lawyer as a "hand grenade who is going to blow everybody up," according to a former White House aide.

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The aide, Fiona Hill, testified for more than 10 hours on Monday as part of the Democrats' impeachment probe into Trump's dealings with Ukraine. She detailed Bolton's concerns to lawmakers and told them that she had at least two meetings with National Security Council lawyer John Eisenberg about the matter at Bolton's request, according to a person familiar with the testimony who requested anonymity to discuss the confidential interview.

Those meetings took place in early July, weeks before a July 25 phone call between Trump and Ukrainian President Volodymyr Zelenskiy, in which Trump urged that Zelenskiy investigate political rival Joe Biden's family and Ukraine's own involvement in the 2016 presidential election.

A whistleblower complaint about that call, later made public, prompted House Speaker Nancy Pelosi to launch the impeachment inquiry. Giuliani is Trump's personal lawyer and was heavily involved in the effort to pressure Ukraine on the investigations.

Hill, a top adviser on Russia, also referred to U.S. ambassador Gordon Sondland and acting White House chief of staff Mick Mulvaney, the person said, telling the three committees leading the investigation that Bolton also told her he was not part of "whatever drug deal Sondland and Mulvaney are cooking up," an apparent reference to talks over Ukraine.

She quoted Bolton, whom Trump forced out last month, as saying in one conversation that Giuliani was "a hand grenade who's going to blow everybody up."

Sondland is expected to appear for a deposition under subpoena Thursday and will certainly be asked about those talks. He's expected to tell Congress that a text message released earlier this month reassuring another envoy that there was no quid pro quo in their interactions with Ukraine was based solely on what Trump told him, according to a person familiar with his coming testimony.

The cache of text messages was provided by one of the inquiry's first witnesses, former Ukrainian envoy Kurt Volker, and detailed attempts by the diplomats to serve as intermediaries around the time Trump urged Zelenskiy to start the investigations into a company linked to Biden's son.

Hill also told the investigators that she had strongly and repeatedly objected to the ouster earlier this year of former ambassador to Ukraine Marie Yovanovitch, according to the person familiar with the testimony. Yovanovitch testified to the impeachment investigators Friday that Trump pressured the State Department to fire her.

The interview is one of what could eventually become dozens of closed-door depositions in the impeachment probe. There are five more scheduled this week, mostly with State Department officials, though it is unclear if they will all appear after Trump declared he wouldn't cooperate with the probe. On Tuesday, Deputy Assistant Secretary of State George Kent is scheduled to testify.

While interviews have focused on the interactions with Ukraine, the probe could broaden as soon as next week to include interviews with White House budget officials who may be able to shed light on whether military aid was withheld from Ukraine as Trump and Giuliani pushed for the investigations.

The three committees leading the probe are seeking interviews next week with Russell Vought, acting director of the Office of Management and Budget, and Michael Duffey, another OMB official who leads national security programs, according to a person familiar with those requests. That person wasn't authorized to discuss the invitations and requested anonymity.

The packed schedule of interviews comes as Democrats are methodically working to pin down the details of Trump's pressure on Zelenskiy. Once Democrats have completed the probe and followed any other threads it produces, they will use their findings to help determine whether to vote on articles of impeachment. Pelosi said she wants the committees to move "expeditiously."

Michael McKinley, a former top aide to Secretary of State Mike Pompeo who resigned last week, is scheduled to testify Wednesday. McKinley, a career foreign service officer and Pompeo's de facto chief of staff, resigned Friday, ending a 37-year career.

The committees are also scheduled to talk to Ulrich Brechbuhl, a State Department counselor, on Thursday. On Friday, the lawmakers have scheduled an interview with Laura Cooper, Deputy Assistant Secretary of Defense for Russia, Ukraine and Eurasia. It is unclear if any of those officials will show up after Trump's vow of non-cooperation.

Because of the Trump administration's edict, the Democrats have been subpoenaing witnesses as they arrived for their interviews — a move sometimes known as a "friendly" subpoena that could give the witnesses additional legal protection as they testify. Both Yovanovitch and Hill received subpoenas the mornings of their testimony.

One witness who may not be called before Congress is the still-anonymous government whistleblower who touched off the impeachment inquiry.

Top Democrats say testimony and evidence coming in from other witnesses, and even the Republican president himself, are backing up the whistleblower's account of what transpired during Trump's July 25 phone call with Zelenskiy. Lawmakers have grown deeply concerned about protecting the person from Trump's threats and may not wish to risk exposing the whistleblower's identity.

House Intelligence Committee Chairman Adam Schiff, D-Calif., said Sunday it "may not be necessary" to reveal the whistleblower's identity as the House gathers evidence. He said Democrats "don't need the whistleblower, who wasn't on the call, to tell us what took place on the call."

Trump showed no signs of backing down.

"Adam Schiff now doesn't seem to want the Whistleblower to testify. NO!" the Republican president tweeted Monday. "We must determine the Whistleblower's identity to determine WHY this was done to the USA."

Republican lawmakers have aimed their ire at Democrats and the process, saying Pelosi should hold a vote to begin the inquiry and hold the meetings out in the open, not behind closed doors.

"The tragedy here and the crime here is that the American people don't get to see what's going on in these sessions," said Ohio Rep. Jim Jordan, the top Republican on the House Oversight and Reform panel.

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