WASHINGTON – More than a dozen witnesses have been called before a trio of House committees and questioned for hours about President Donald Trump and Ukraine.

The witnesses include diplomats and White House officials with knowledge of Trump's dealings with Ukraine. Each has provided new details as part of the quickly moving impeachment inquiry examining whether Trump abused his power as president in asking Ukraine to investigate political foes while dangling military aid for the country and a White House meeting. Their testimonies combined span about 100 hours.

Almost a dozen of these witnesses have also appeared publicly before the House Intelligence Committee.

Here are the 17 witnesses who have been interviewed behind closed doors by the House Intelligence, Foreign Affairs and Oversight committees or later testified publicly, why they matter in the impeachment saga and what we know about their testimony.

Oct. 3: Former U.S. special envoy to Ukraine Kurt Volker

Why he matters: A career State Department official, Volker worked with Trump's personal lawyer Rudy Giuliani and various White House officials to set up Trump's phone call July 25 with Ukrainian President Volodymyr Zelensky and a potential White House visit. This happened as military aid for the country was on pause.

What he told lawmakers:Volker said he never saw anything that made him believe there was a quid pro quo with Ukraine and said he felt coordinating with Giuliani could change Trump from his negative feelings about Ukraine. His testimony made clear the influence Giuliani had over policy, including Giuliani dictating a statement to Volker that he wanted the Ukrainians to issue on corruption.

In his public testimony Nov. 19, Volker told lawmakers he was not aware of or knowingly took part in any efforts to pressure Kyiv to open an investigation into former Vice President Joe Biden. At the time, he told lawmakers, he did not realize investigations into the Bidens and into Burisma had been conflated.

Oct. 11: Former Ukraine ambassador Marie Yovanovitch

Why she matters: A career diplomat, Yovanovitch was forced out of her role.

What she told lawmakers: Yovanovitch was confused by her ouster and the comments made by conservatives and Trump, who called her "bad news." She said she raised concerns about the shadow campaign pushed by Giuliani and how it ran counter to U.S. policy.

In her Nov. 15 public hearing, Yovanovitchhow she felt after being ousted as ambassador amid the "smear" campaign against her.

When asked how it felt to be told to leave Ukraine as quickly as possible, she said it felt “terrible, honestly. After 33 years of service to our country it was terrible. It's not the way I wanted my career to end.”

Oct. 14: Trump's former Russia expert Fiona Hill

Why she matters: Hill worked for years on the National Intelligence Council and as Trump's senior adviser on the Kremlin and Europe. She held a key role in U.S. policy in Ukraine and was part of several meetings where she expressed concerns over the shadow policy led by Giuliani and White House acting Chief of Staff Mick Mulvaney.

What she told lawmakers: Hill told lawmakers national security adviser John Bolton likened the policy in Ukraine to a "drug deal" and called Giuliani a "hand grenade" who was going to blow everyone up, according to The New York Times and NBC News.

In her Nov. 21 public testimony, Hill slammed the "fictional narrative" of Ukrainian interference in the 2016 election. Congressional Republicans have argued Ukraine colluded with Democrats to hurt Trump in the election.

Oct. 15: State Department Ukraine-Russia expert George Kent

Why he matters: Kent serves as a deputy assistant secretary at the State Department.

What he told lawmakers: Kent told lawmakers he raised red flags about Giuliani's efforts to pressure Ukraine to investigate former Vice President Joe Biden more than six months ago.

In his public testimony, Kent said the Ukrainian oligarch owner of gas company Burisma should be investigated over allegations of paying a bribe to kill a probe into the company.

Oct. 16: Michael McKinley, ex-adviser to Secretary of State Mike Pompeo

Why he matters: McKinley boasted a career that spanned decades at the State Department and resigned just before his testimony because of low morale at the department and because he said Pompeo did not stick up for career employees, such as Yovanovitch.

What he told lawmakers: McKinley didn't oversee issues related to Ukraine, so his testimony did not deal with the core allegations against Trump. He outlined his concerns about the ouster of Yovanovitch and said he was troubled that the State Department did not have her back and that she and the department were being politicized.

Oct. 17: U.S. ambassador to the European Union Gordon Sondland

Why he matters: A businessman and major Trump donor, Sondland was in communication with the president and Giuliani and attempted to get Ukrainians to investigate several political matters, according to witnesses.

What he told lawmakers: Sondland amended his original testimony and told lawmakers he communicated a quid pro quo to a Ukrainian official, linking military aid for Ukraine to a public statement committing to investigations Trump and Giuliani wanted.

In his Nov. 20 public testimony, Sondland gave more details not previously included in his closed-door interview, telling lawmakers that "everyone was in the loop" on the pressure campaign – including Secretary of State Mike Pompeo and Vice President Mike Pence.

Oct. 22: Bill Taylor, top U.S. diplomat in Ukraine

Why he matters: Taylor voiced concerns about conditioning military aid and a White House meeting on political investigations.

What he told lawmakers: Taylor directly tied Trump and his allies with a quid pro quo. He said Trump made the order to pause military aid for Ukraine, and it was his "clear understanding" that "security assistance money would not come until the (Ukrainian) president committed to pursue the investigation," according to a transcript of his testimony.

In his public hearing, Taylor gave new testimony saying one of his aides overheard a call between Trump and Sondland in which Trump asked Sondland about "investigations," which caused Trump to deny the conversation.

Oct. 23: Defense official Laura Cooper

Why she matters: Cooper serves as the deputy assistant secretary of defense for Russia, Ukraine and Eurasia.

What she told lawmakers: Behind closed doors, Cooper gave lawmakers further details about the withholding of aid.

In her Nov. 21 public testimony, Cooper provided new details about emails from the Ukrainians showing they had concerns about security assistance on July 25 – the day of Trump's infamous call with Zelensky.

Oct. 26: State Department official Philip Reeker

Why he matters: Reeker serves as the acting assistant secretary of state in the Bureau of European and Eurasian Affairs.

What he told lawmakers: According to The Wall Street Journal, he discussed with lawmakers failed efforts to help Yovanovitch.

Oct. 29: Ukraine expert Lt. Col. Alexander Vindman

Why he matters: Vindman is one of several officials who listened to Trump's phone call with Zelensky on July 25. He served as the White House's top Ukraine expert.

What he told lawmakers: Vindman said in prepared remarks he twice reported concerns to superiors that the president and those working for him linked foreign aid to Ukraine with political investigations. He said he worried the efforts undermined U.S. national security.

In his public testimony on Nov. 20, Vindman said he reported his concerns about a July 10 meeting between Sondland and Ukrainian officials, as well as Trump's July 25 call with Zelensky, out of a sense of duty.

"I was concerned by the call," Vindman said. "What I heard was inappropriate."

Oct. 30: State Department official Catherine Croft

Why she matters: Croft worked for Volker at the State Department and has expertise on Ukrainian issues. She focused on arms sales and security assistance for the country as it fended off Russia.

What she told lawmakers: According to a copy of her opening statement, Croft told lawmakers she received calls from a lobbyist trying to oust Yovanovitch. She said she learned that aid was put on hold stemming from an order from the president.

Oct. 30: State Department official Christopher Anderson

Why he matters: Anderson worked for Volker at the State Department and was present for at least one meeting where political investigations were discussed.

What he told lawmakers: In his opening statement, Anderson said Giuliani's efforts were discussed at a Ukraine strategy meeting over the summer. At that meeting, Bolton said Giuliani's efforts "could be an obstacle." Anderson said he believed it was important to not request specific investigations from the Ukrainians.

Oct. 31: NSC official Timothy Morrison

Why he matters: Morrison is the top Russia and European adviser to Trump's National Security Council and was cited by multiple witnesses in conversations about a quid pro quo. He is a political appointee and not a career official.

What he told lawmakers: Morrison confirmed testimony given by Taylor that outlined a quid pro quo, basically halting aid until Ukraine committed to investigations. Morrison testified that he didn't believe Trump's call July 25 was illegal.

In his Nov. 19 public testimony, Morrison said fellow NSC staffer Fiona Hill warned him of the “Gordon problem” regarding Sondland’s dealings in Ukraine.

Nov. 6: State Department's David Hale

Why he matters: Hale is the third highest-ranking official at the State Department.

What he told lawmakers: Hale told lawmakers about the political considerations in dismissing Yovanovitch and how those decisions affected military aid for Ukraine.

In his Nov. 20 public testimony, Hale said the State Department's decision not to issue a statement of support for Yovanovitch could only be made by someone "more senior to me. The Secretary most likely would have been the person."

Nov. 7: Pence aide Jennifer Williams

Why she matters: A foreign policy aide to Vice President Mike Pence, Williams was one of a handful of officials who listened to Trump's July 25 call with the Ukrainian president.

What she told lawmakers: In her closed-door interview, Williams told lawmakers and staff it was “folly” to withhold military aid to Ukraine and that the call was “unusual.”

In her Nov. 19 public testimony, Williams said she found Trump’s July 25 phone call “unusual” because of the discussion of “a domestic political matter.”

Nov. 15: State Department official David Holmes

Why he matters: In his public testimony, Taylor told lawmakers about a call one of his aides, later identified to be Holmes, overheard between Trump and Sondland in which Trump asked Sondland about "investigations."

What he told lawmakers: Holmes confirmed he overheard the call and that he "heard President Trump ask, 'So, he's gonna do the investigation?' Ambassador Sondland replied that 'he's gonna do it,' adding that President Zelensky will do 'anything you ask him to.'"

In his Nov. 21 public testimony, Holmes said U.S. foreign policy towards Ukraine "became overshadowed by a political agenda being promoted" by Trump's personal lawyer, Rudy Giuliani, and a "cadre of officials operating with a direct channel to the White House."

Nov. 16: White House official Mark Sandy

Why he matters: Sandy a White House official in the Office of Management and Budget, is the first OMB official to testify in the inquiry.

What he told lawmakers: Documents presented during the deposition of Deputy Assistant Secretary of Defense Laura Cooper showed that Sandy signed one of the "apportionment" letters in July 2019 halting security assistance to Ukraine.

Democrats allege Trump tied the aid and a White House meeting to requests for investigations that benefited him politically.