WASHINGTON - The House's impeachment inquiry is turning attention toward former national security adviser John Bolton, after a former top aide delivered explosive testimony Monday describing how he was infuriated by a shadow operation being conducted by the president's allies and his lawyer in Ukraine to dig up dirt on the president's political rivals.

Fiona Hill, who was the National Security Council's top Russia and Europe adviser under Bolton, told investigators that Bolton likened President Trump's lawyer Rudy Giuliani, a driving force behind the efforts, to a "hand grenade," according to two people familiar with her testimony.

Hill also testified that Bolton wanted to make clear he was not involved and very opposed to what he described as the "drug deal" between the White House's acting chief of staff, Mick Mulvaney, and U.S. Ambassador to the European Union Gordon Sondland, who were also involved in the effort, the people said.

Some lawmakers and aides are already talking privately about a need to call Bolton to testify, although they deferred to House Intelligence Committee Chairman Adam Schiff, D-Calif., and House Speaker Nancy Pelosi, D-Calif., on next steps.

Trump ousted Bolton last month after a rocky relationship in which the two men clashed over policy toward North Korea, Iran and Afghanistan, among other issues. The president disparaged Bolton, saying he made "some very big mistakes."

Bolton, who initially declined to comment, said in a text: "I will have my say in due course." He is reportedly writing a book.

The revelations about Bolton's stance, many of which were first reported by the New York Times, paint a picture of a White House bitterly divided not just over Ukraine, which has long been reliant on military aid and political support from the United States as it fights Russian-backed separatists in its eastern territories, but also over which political appointees were calling the shots on foreign policy: the experienced national security staff, or a group of Trump loyalists and Giuliani.

According to one person familiar with Hill's testimony, Bolton was so alarmed by the efforts of Giuliani, Sondland and Mulvaney to circumvent the NSC and diplomatic corps that he dispatched her to raise the concern with White House lawyers.

The order came after then-special envoy for Ukraine Kurt Volker, Bolton, Sondland, Hill and Energy Secretary Rick Perry met in early July. During the meeting, Sondland blurted out to the other officials present that there were "investigations that were dropped that need to be started up again" in Ukraine, according to a U.S. official familiar with the matter. The officials understood him to be referring to Burisma, a Ukrainian energy company, and former vice president Joe Biden's son Hunter Biden, who sat on its board.

Bolton went "ballistic" after the meeting, the official said.

Hill herself got into a confrontation with Sondland over his involvement in Ukrainian affairs, according to one person familiar with her testimony, as Ukraine is not in the European Union and thus not part of his ambassadorial portfolio. Sondland said he had been put in charge by Trump, the person said - something Hill likened to the bravado of when Alexander Haig, then secretary of state, said he was in charge after a 1981 assassination attempt on President Ronald Reagan.

Sondland, who obeyed State Department orders not to show up for a planned deposition last week, is expected to testify in the House's impeachment inquiry Thursday under subpoena. Text messages provided to the panel byt Volker showed it was Sondland who defended the president in early September, when other diplomats expressed concern that U.S. military assistance was being withheld from Ukraine to push Ukrainian leaders to conduct a politically motivated investigation of Burisma.

Hunter Biden served on Burisma's board for five years; Joe Biden is currently making a 2020 White House bid.

Hill's testimony about Bolton may also ensnare Mulvaney in the impeachment investigation. Trump told Mulvaney in mid-July to hold back almost $400 million in congressionally approved military aid for Ukraine. That order came the same week that Hill resigned from the NSC; it also took place one week before Trump spoke with Ukrainian President Volodymyr Zelensky by phone on July 25, when he appeared to pressure Zelensky to investigate the Bidens and purported Ukrainian interference in the 2016 U.S. presidential election.

The panels have already subpoenaed Giuliani for documents - a subpoena he has promised to disregard.