WASHINGTON - It was a long and exhausting day in Washington Tuesday, where testimony in the House of Representatives impeachment inquiry into President Donald Trump lasted more than 11 hours.

Lawmakers on the Intelligence Committee heard from four witnesses – three of whom directly listened to Trump's July 25 telephone call with Ukrainian President Volodymyr Zelenskiy, when he asked Zelenskiy to investigate Democratic presidential candidate Joe Biden.

Army Lt. Col. Alexander Vindman, the top Ukraine expert on the White House's National Security Council, said Trump's call was "inappropriate" and "improper."

National Security Council aide Lt. Col. Alexander Vindman testifies before the House Intelligence Committee on Capitol Hill in Washington, Nov. 19, 2019.

"Frankly, I couldn't believe what I was hearing. It was probably an element of shock that maybe, in certain regards, my worst fear of how our Ukrainian policy could play out was playing out," he said.

Vindman said Trump's request to investigate Biden "had nothing to do with U.S. national security" interests and was not part of the talking points Vindman and others prepared for Trump ahead of the call.

Vindman appeared before the committee in his full Army dress uniform, including a decoration for Iraqi war wounds.

Donald Trump Jr. tweeted that Vindman was a "low-level partisan bureaucrat." Vindman called character attacks on public servants testifying "reprehensible,” while stressing that he is 100% nonpartisan and was only there to give the facts.

Jennifer Williams, special adviser to Vice President Mike Pence for European and Russian affairs, testifies before a House Intelligence Committee hearing as part of the impeachment inquiry into President Donald Trump on Capitol Hill, Nov. 19, 2019.

Jennifer Williams, a foreign affairs adviser to Vice President Mike Pence, also testified Tuesday. She heard the call and said the Trump request for a Biden probe was "unusual" because it involved a "domestic political matter" and not foreign policy

Williams said in her 14 years as a foreign service officer, she has heard a lot of presidential phone calls, but nothing like what Trump has asking for.

After a break, the committee heard from two more witnesses.

Ambassador Kurt Volker, the former U.S. special envoy to Ukraine, had said in closed-door testimony that he did not see a link between Trump pressuring Ukraine for a Biden probe and Trump withholding nearly $400 million in military aid.

Volker said in Tuesday's testimony that after hearing other witnesses and seeing the transcript of the July 25 phone call, he now believes there was a connection. He said if he had seen things differently earlier, he would have raised objections.

Ambassador Kurt Volker, former special envoy to Ukraine, testifies before the House Intelligence Committee on Capitol Hill in Washington, Nov. 19, 2019.

Volker also said he failed to realize that when the White House asked for a corruption investigation into the Ukrainian gas company Burisma, it really meant an investigation into Biden, whose son, Hunter, sat on the company’s board.

Volker defended Biden, saying he has known the former U.S. vice president for more than 20 years and called allegations that Biden had a financial motive in Ukraine not credible.

Volker said he believes Trump has a deeply negative view of Ukraine as a hopelessly corrupt country full of "terrible people" who tried to destroy him by meddling in the 2016 election on behalf of the Democrats. Volker said Trump's views were rooted in the past, before Zelenskiy took over, and egged on by Rudy Giuliani, Trump's personal lawyer, who insists Biden is corrupt and leaned hard on the Ukrainians to investigate.

Tim Morrison, who was the top director for European affairs on the National Security Council, testified Tuesday that he did not hear anything in Trump's July phone call that he would call illegal.

Tim Morrison, a former official at the National Security Council, listens as he testifies before the House Intelligence Committee on Capitol Hill in Washington, Nov. 19, 2019, during an impeachment hearing.

But Morrison said he does not think investigating Biden should be a "policy objective."

Democrats want to know why a complete word-for-word transcript of the July 25 call was put in a highly secure White House location, reserved only for the most sensitive of documents.

Morrison called it a "mistake" and an "administrative error."

The impeachment inquiry was sparked by an intelligence community whistleblower who was disturbed by the July 25 phone call and informed the intelligence inspector general.

Republican lawmakers have been fishing for the name of the whistleblower during the hearings and tried to get Vindman to reveal the name, believing he knows who it is. But Intelligence Chairman Adam Schiff has said he will not permit such tactics, reminding the Republicans that federal law protects the whistleblower's identity.

Trump and the Republicans allege that when Biden was vice president, he threatened to withhold loan guarantees to Ukraine unless prosecutors stopped a corruption probe into Burisma.

No evidence of wrongdoing by the Bidens has surfaced, and the allegations of Ukrainian election interference are based on a debunked conspiracy theory.

Democrats are focused on whether Trump froze military aid to Ukraine in exchange for Zelenskiy publicly committing to an investigation of the Bidens and Democrats. Some Democrats accuse Trump of bribery — an impeachable crime.

Trump calls the hearings a huge scam and a witch hunt, insisting he did nothing wrong.

U.S. Ambassador to the European Union Gordon Sondland, left, walks to a secure area of the Capitol to testify as part of the House impeachment inquiry into President Donald Trump, Oct. 28, 2019, in Washington.

Three more witnesses will testify Wednesday, including U.S. Ambassador to the European Union Gordon Sondland. Sondland was in frequent contact with Trump and other administration officials about Ukraine policy and pressure to carry out the investigations Trump demanded.