Watch: Trump impeachment witnesses testify about John Bolton and Rudy Giuliani roles in Ukraine scandal

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David Holmes, an American diplomat stationed in Ukraine, testifies before the House Intelligence Committee in the fifth round of public impeachment hearings Thursday, alongside former National Security Council aide Fiona Hill.

With the hearings, the House is investigating whether the president abused his powers by withholding aid to Ukraine in order to pressure the country's newly elected president to launch investigations into former Vice President Joe Biden and his son Hunter. Biden is a top contender in the 2020 Democratic presidential race.

Holmes described in closed-door testimony the shock he felt while watching Sondland, the U.S. ambassador to the European Union, call President Donald Trump from a personal cellphone and discuss the prospect of Ukraine launching investigations into Biden.

"I've never seen anything like this, someone calling the president from a mobile phone at a restaurant, and then having a conversation of this level of candor, colorful language," Holmes said according to a testimony that was released Monday. "There's just so much about the call that was so remarkable that I remember it vividly."

Holmes is a last-minute addition to the public impeachment hearings.

Hill in her testimony said that former national security advisor John Bolton was disturbed by Trump personal lawyer Rudy Giuliani's involvement in the conversations with Ukraine. Trump instructed a small group of people — special enjoy to Ukraine Kurt Volker, Energy Secretary Rick Perry and Sondland — to collaborate with Giuliani.

"Rudy Giuliani is a hand grenade that is going to blow everybody up," Hill said Bolton told her.

Hill left the NSC before the July 25 phone call between Trump and Ukraine President Volodymyr Zelenskiy, in which Trump asked him for "a favor."

In bombshell testimony Wednesday, Sondland said Trump directed Giuliani to pursue a "quid pro quo" with Ukraine. He also implicated other senior Trump administration officials in the scheme, including Vice President Mike Pence, Secretary of State Mike Pompeo and acting White House chief of staff Mick Mulvaney.

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