Fiona Hill, former Special Assistant to U.S. President Donald Trump and Senior Director for European and Russian Affairs on the United States National Security Council, arrives on Capitol Hill on October 14, 2019 in Washington, DC.

WASHINGTON — President Donald Trump's former top Russia expert, Fiona Hill, vividly described alarms raised by White House national security professionals about his Ukraine policy, according to newly released testimony to the impeachment investigators.

Hill left the White House National Security Council shortly before the July 25 call during which Trump asked Ukrainian President Volodymyr Zelensky for "a favor."

Her testimony, released Friday, offered a gripping account of the rising tensions in the spring and summer between members of the NSC, led by former National Security Advisor John Bolton, and the small group of individuals whom Trump directed to run a parallel foreign policy aimed at getting Ukraine to launch investigations on Democrat Joe Biden and his son.

That small group consisted of Kurt Volker, Trump's special envoy to Ukraine, Gordon Sondland, Trump's ambassador to the European Union, and Energy Secretary Rick Perry. The president instructed the three to work closely with his personal lawyer, Rudy Giuliani.

Hill testified that inside the White House, acting chief of staff Mick Mulvaney also played an instrumental role.

In her testimony, Hill described Bolton's growing frustration with Giuliani's involvement in back channel communications with Ukraine's newly elected government. "Rudy Giuliani is a hand grenade that is going to blow everybody up," Hill quoted Bolton as having told her.

The simmering tensions boiled over on July 10, Hill said, during a heated exchange between Bolton and Sondland at the White House. The conversation so angered Bolton that he directed Hill to notify the chief lawyer for the NSC about the pressure campaign on Ukraine to pursue investigations that Trump and Giuliani wanted.

Hill also said Bolton told her to convey to the NSC attorney that Bolton was, "not part of whatever drug deal Sondland and Mulvaney are cooking up."

House investigators have invited Bolton to testify, but his attorney has signaled that Bolton would appear only if a judge ruled that he was required to comply with any subpoena.

According to the testimony of current and former national security officials, Giuliani and Trump wanted Ukraine to announce two investigations. One was a probe into a Ukrainian energy company, Burisma, on whose board Biden's son Hunter once sat. The second investigation Trump wanted, according to Sondland and others, was into alleged interference by Ukraine in the 2016 presidential election.

The allegations that Ukraine interfered in the U.S. presidential election in 2016 are unsubstantiated, and appear to be based on a conspiracy theory that serves as a counter-narrative to former special counsel Robert Mueller's conclusion that Russia disrupted the U.S. race in Trump's favor.

Hill was the first former White House official to testify in the House impeachment inquiry, initially appearing on Oct. 14.

The House impeachment probe focuses on whether Trump abused the power of his office in his attempt to pressure Ukraine for personal political gain, and if so, whether those actions meet the standard for "high crimes and misdemeanors" deserving of impeachment and, potentially, removal from office.