An anonymous whistleblower complaint in September highlighted potential abuse of power by the president when he telephoned Ukraine's leader and asked him to investigate former vice president Joe Biden, Trump's potential 2020 election rival.
A source with knowledge of her testimony added that Williams did not say she believed the two were connected, but simply expressed that as a possibility.
An attorney for Bolton, Charles Cooper, said his client has not received a subpoena. Cooper had said Bolton wouldn't appear without one.
The top USA diplomat in Ukraine, Bill Taylor, told House impeachment investigators last month that President Donald Trump directed officials to tie military aid to Ukraine to demands that the country open political advantageous probes, according to a transcript of his testimony made public Wednesday.
Trump has blasted the House inquiry as a witch hunt and accused Democrats of unfairly targeting him in hope of reversing his surprise victory in the 2016 presidential election.
"Don't be fooled. Public hearings are not the same as FAIR hearings", McCarthy tweeted.
David Hale, the third most senior official at the state department, has begun his testimony at congressional impeachment hearings, where he will reportedly seek to explain the agency's failure to defend the ambassador to Ukraine as an act of realpolitik.
Kent also raised concerns about the Trump administration's recall of its Ukraine ambassador, Marie Yovanovitch.
The top USA diplomat in Ukraine, William Taylor, was "embarrassed" when he was questioned by Ukraine this summer about why hundreds of millions of dollars in foreign military aid to fend off Russian aggressions were being withheld. Trump has directed his employees not to cooperate with the probe.
Democrats had said they had enough material to move forward with public impeachment hearings, which would be a likely prelude to articles of impeachment - formal charges - against Trump being brought to a vote in the House. If required to appear, she will answer the Committees' questions.
Still, Democrats have indicated they think they already have ample testimony about Trump's conduct on Ukraine. A slew of current and former officials from the State Department and White House have already appeared and largely corroborated the same narrative - that Trump had delegated his lawyer, Rudy Giuliani, to guide Ukraine policy and that the two men were focused on pressuring Ukraine as the administration withheld military aid from the country.
On July 25, the day of Trump's infamous phone call with Zelensky, Volker wrote Yermak that he "heard from White House - assuming President Z convinces trump he will investigate / 'get to the bottom of what happened" in 2016, we will nail down date for visit to Washington'.
Leading up to his discovery of evidence there was a quid pro quo, Taylor said he began to raise concerns with USA officials, including Secretary of State Mike Pompeo, about why the aid was not being released.
Lawmakers asked if he was aware that "quid pro quo" meant "this for that". But we would like to invite readers who can afford to do so, and for whom The Times of Israel has become important, to help support our journalism by joining The Times of Israel Community.