(Reuters) - The U.S. House of Representatives’ impeachment inquiry into President Donald Trump shifts into higher gear this week with a parade of officials facing questioning by lawmakers over Trump’s dealings with Ukraine.

At issue is whether Trump used nearly $400 million in security assistance to Ukraine to pressure the country to investigate his political rival Joe Biden, a leading Democratic candidate opposing Trump’s reelection bid, and his son Hunter.

Following are the nine witnesses scheduled to testify in what is the second week of the televised hearings.

Tuesday, Nov. 19

9 A.M. EST (1400 GMT)

JENNIFER WILLIAMS, an aide to U.S. Vice President Mike Pence

Williams was on a July 25 phone call between Trump and Ukraine President Volodymyr Zelenskiy that is at the heart of the inquiry. Behind closed doors, she testified that Trump’s insistence that Ukraine carry out politically sensitive investigations “struck me as unusual and inappropriate.”

Trump has taken to Twitter to criticize Williams, who has been working temporarily in Pence’s office since April 1.

LIEUTENANT COLONEL ALEXANDER VINDMAN, director for European Affairs at the National Security Council

Vindman was also among U.S. officials monitoring Trump’s July 25 call. According to a transcript of his testimony behind closed doors, Vindman said he had no doubt that Trump demanded that Ukraine investigate the Bidens in return for an Oval Office visit for Zelenskiy.

Trump has also attacked Vindman on Twitter, calling him a “Never Trump Witness,” which raised questions about fallout in his military career.

Jennifer Williams, a special adviser to Vice President Mike Pence for European and Russian affairs and Alexander Vindman, director for European Affairs at the National Security Council, are sworn in to testify before a House Intelligence Committee hearing as part of the impeachment inquiry into U.S. President Donald Trump on Capitol Hill in Washington, U.S., November 19, 2019. REUTERS/Jonathan Ernst

2:30 P.M EST (1930 GMT)

KURT VOLKER, former U.S. special envoy to Ukraine

In his closed-door testimony, Volker detailed what he described as the role of Trump’s personal lawyer, Rudy Giuliani, as a conduit between Washington and Kiev. Volker, along with U.S. Ambassador to the European Union Gordon Sondland and Energy Secretary Rick Perry, were known as the “three amigos,” responsible for Trump’s unofficial channel to Ukraine under Giuliani’s direction.

Volker resigned from his position as special envoy on Sept. 27 because of the impeachment inquiry.

TIM MORRISON, former White House aide with the National Security Council focused on Europe and Russia policy

Morrison, who was also on the July 25 call, said in his closed-door deposition that he was uncomfortable with Ukraine’s leader becoming involved in U.S. politics. Morrison said he reported the call to legal counsel because he was concerned it could damage U.S.-Ukrainian relations if it were leaked. He resigned a day before his deposition.

Wednesday, Nov. 20

9 A.M. EST (1400 GMT)

GORDON SONDLAND, U.S. Ambassador to the European Union

Sondland, another of the “three amigos,” said in amended testimony released on Nov. 5, that he had told a Ukrainian official his country would likely not get the $391 million in security assistance unless it pursued investigations demanded by Trump. In previous testimony, he had said he was not aware of any preconditions on the aid.

2:30 P.M. EST (1930 GMT)

LAURA COOPER, deputy assistant secretary of defense for Russian, Ukrainian and Eurasian affairs

In her closed-door deposition, Cooper detailed confusion and concern among national security officials after the White House blocked the security assistance for Ukraine without explanation. She said the military was concerned that denying U.S. assistance would make it harder for Kiev to negotiate with Moscow.

DAVID HALE, undersecretary of state for political affairs

The third-highest-ranked official at the U.S. State Department, Hale is expected to testify about the circumstances around the recall of U.S. Ambassador to Ukraine Marie Yovanovitch in May. Yovanovitch’s ouster preceded the opening of Trump’s unofficial channel to Ukraine under Giuliani, according to other witnesses.

Thursday, Nov. 21

9 A.M. EST (1400 GMT)

FIONA HILL, former National Security Council senior director for Europe and Russia

In her closed-door testimony, Hill said Sondland told the Ukrainians that Zelenskiy would only be invited to the White House for a meeting with Trump if he opened investigations into Biden. Hill also testified that she was concerned about the role Giuliani played in dealings with Ukraine.

DAVID HOLMES, political counselor at the U.S. Embassy, Kiev, Ukraine

Holmes, an aide to acting U.S. ambassador to Ukraine William Taylor, gave a last-minute deposition behind closed doors on Nov. 15, saying he overheard Trump ask Sondland about the status of “investigations” into his political rivals during a phone call on July 26, a day after the call with Zelenskiy.