WASHINGTON – House Democrats have escalated their impeachment inquiry into President Donald Trump's pressuring of the Ukrainian government to open an investigation into former Vice President Joe Biden and his son Hunter Biden, subpoenaing White House and administration officials over documents and testimony related to Trump's contacts with Ukraine.

The Trump administration has said it would not cooperate with the impeachment inquiry, setting up a clash between the legislative and executive branches.

If the White House does not comply with a subpoena, the House could file a civil lawsuit, or hold people in contempt of Congres. It is unclear what ability Congress has to enforce its subpoenas, though.

Here are all of the officials or individuals who have been subpoenaed or asked to provide information to congressional investigators since House Democrats announced their formal impeachment inquiry on Sept. 24.

Timothy Morrison (Oct. 31)

Timothy Morrison, the NSC senior director for Europe and Russia, was subpoenaed by the House Intelligence Committee following "an attempt by the White House to direct Timothy Morrison not to appear for his scheduled deposition," according to an official working on the inquiry.

Morrison testified about the withholding of military aid to Ukraine. William Taylor, America's top diplomat in Ukraine, identified him as a key witness.

Catherine Croft and Christopher Anderson (Oct. 30)

Catherine Croft and Christopher Anderson, both State Department officials appeared under subpoena for their interviews with the House Intelligence, Oversight, and Foreign Affairs Committees.

According to their opening statements, Croft and Anderson's testimonies could be helpful to Democrats hoping to learn more about the dual strategies that the U.S. was taking on Ukraine — the official one and the operation that multiple officials have told lawmakers hinged on the country investigating a political rival of the president.

Lt. Col. Alexander Vindman (Oct. 29)

National Security Council expert on Ukraine appeared voluntarily under subpoena for his interview with the House Intelligence, Oversight, and Foreign Affairs Committees.

According to his opening statement, Vindman told the committees he was "concerned" by Trump's pressuring of Ukraine to open investigations and reported the issue to superiors.

Former Deputy National Security Adviser Charles Kupperman (Oct. 28)

Former Deputy National Security Adviser Charles Kupperman defied a subpoena and did not show up for his interview with the committees leading the impeachment investigation.

Kupperman has filed a federal lawsuit asking a judge to decide whether he should testify because he's worried any decision he makes "will inflict grave Constitutional injury on either the House or the President."

Acting Assistant Secretary of State Philip Reeker (Oct. 26)

Acting Assistant Secretary of State in the Bureau of European and Eurasian Affairs Philip Reeker was subpoenaed "In light of an attempt by the State Department to direct Ambassador Philip Reeker not to appear for his scheduled deposition, and efforts by the State Department to also limit any testimony that does occur," according to an official working on the impeachment inquiry.

The Associated Press reported Reeker answered questions for eight hours and was concerned about efforts led by Trump to oust former Ambassador to Ukraine Marie Yovanovitch.

Deputy Assistant Secretary Laura Cooper (Oct. 23)

Deputy Assistant Secretary of Defense for Russia, Ukraine, and Eurasia Laura Cooper was subpoenaed to take part in a closed-door session after the Department of Defense attempted to block her from testifying. She hired a private lawyer to represent her.

Congressional committees asked her about the withholding of military aid to Ukraine, though her testimony was delayed after House Republicans staged a protest at the secure room in the Capitol where her deposition was being held.

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Ambassador William Taylor (Oct. 22)

Lawmakers issued a subpoena to compel the testimony of William Taylor, the top American diplomat in Ukraine, after the State Department attempted to block his testimony.

Taylor gave what House Democrats called "explosive" testimony that linked military aid and a meeting between Trump and Zelenksy to the opening of investigations related to Hunter Biden.

Deputy Assistant Secretary George Kent (Oct. 15)

Lawmakers issued a subpoena to compel George Kent's testimony on Oct. 15 after the State Department attempted to block his testimony.

Kent, who serves as a deputy assistant secretary at the State Department's European and Eurasian Bureau, said he raised red flags about Giuliani's effort to pressure Ukraine.

Fiona Hill (Oct. 14)

The House Oversight, Intelligence, and Foreign Affairs committees subpoenaed Fiona Hill, who had been the National Security Council's senior director for Europe and Russia, to compel her testimony before the committees.

Media reports suggest Hill told congressional investigators that then-National Security Adviser John Bolton raised serious concerns about Giuliani's role in Ukraine policy.

Former Ukraine Ambassador Marie Yovanovitch (Oct. 11)

The State Department had attempted to block Marie Yovanovitch, the ambassador to Ukraine, from testifying, so a subpoena was issued to compel her testimony.

In her testimony, Yovanovitch said the Trump administration had pressured the State Department to remove her from her post as part of a "concerted campaign" against her.

Energy Secretary Rick Perry (Oct. 10)

The House Oversight, Intelligence, and Foreign Affairs committees subpoenaed Energy Secretary Rick Perry on Oct. 10 for documents relating to House Democrats' impeachment inquiry.

Congressional Democrats saw Perry as playing a role in "reinforcing the President's stark message to the Ukrainian President," the committees said in a letter.

Giuliani associates Lev Parnas and Igor Fruman (Oct. 10)

Lev Parnas and Igor Fruman, associates of Rudy Giuliani and Ukrainian-born business partners, were subpoenaed for documents relating to the impeachment inquiry on Oct. 10.

They were arrested on Oct. 9 on campaign finance charges and had been among the witnesses congressional investigators had hoped to question. The men helped Giuliani meet a Ukrainian prosecutor as part of Giuliani's push for an investigation into Biden.

Their federal indictment alleges they “conspired to circumvent the federal laws against foreign influence by engaging in a scheme to funnel foreign money to candidates for federal and state office so that the defendants could buy potential influence with candidates, campaigns and the candidates’ governments.”

European Union Ambassador Gordon Sondland (Oct. 8)

Gordon Sondland, the ambassador to the European Union, was scheduled to meet with congressional investigators on Oct. 7 but was directed by the White House not to hold the meeting.

Sondland had been at the center of text messages released as part of Ukrainian envoy Kurt Volker's testimony to Congress last week, and lawmakers on both parties had been looking forward to hearing from Sondland before his testimony was canceled.

"I would love to send Ambassador Sondland, a really good man and great American, to testify, but unfortunately he would be testifying before a totally compromised kangaroo court, where Republican’s rights have been taken away, and true facts are not allowed out for the public," Trump tweeted Tuesday morning.

In response, House Democrats issued a subpoena in an attempt to compel Sondland's testimony and to force the release of text messages or emails on one of his personal devices.

Defense Secretary Mark Esper/The Pentagon (Oct. 7)

House Democrats also sought information from Defense Secretary Mark Esper on any relevant information on the withholding of military aid to Ukraine

They issued a subpoena to Esper and the Department of Defense on Oct. 7.

Acting Office of Management and Budget Director Russell Vought (Oct. 7)

In response to media reports that Trump had ordered then-acting OMB Director Mick Mulvaney to halt military aid to Ukraine, House Democrats issued a subpoena on Oct. 7.

Speaking to reporters outside the White House on Wednesday Vought said he and his office would not be cooperating in the impeachment inquiry.

Acting White House Chief of Staff Mick Mulvaney/The White House (Oct. 4)

House Democrats sent a letter on Oct. 4 to acting White House Chief of Staff Mick Mulvaney carrying subpoenas related to documents relating to the impeachment inquiry. The White House was given a deadline of Oct. 18 to comply.

The subpoena came after the White House ignored several other requests for documents.

Vice President Mike Pence (Oct. 4)

House Democrats issued a request for documents to Vice President Mike Pence relating to the inquiry on Oct. 4, but it appears that Pence's office will not comply with the request.

"Given the scope, it does not appear to be a serious request but just another attempt by the Do Nothing Democrats to call attention to their partisan impeachment," spokeswoman Katie Waldman said in a statement.

Pence's office was given an Oct. 15 deadline to produce the documents. A failure to comply with the request could constitute "evidence of obstruction," Democrats said.

Rudy Giuliani (Sept. 30)

Democrats subpoenaed Trump's personal attorney Rudy Giuliani on Sept. 30 for documents related to his contacts and dealings with Ukrainian officials. Giuliani had played a central role in the controversy around the pressuring of the Ukrainian government to open investigations.

He told The Washington Post on Oct. 8 he would not comply with the subpoena.

"Let them hold me in contempt. We’ll go to court. We’ll challenge the contempt," he told The Washington Post.

Secretary of State Mike Pompeo (Sept. 27)

Democrats subpoenaed Secretary of State Mike Pompeo on Sept. 27 for documents related to Ukraine.

Pompeo failed to comply with the Oct. 4 deadline to produce documents.

"Sadly there have been congressional inquiries that have harassed and abused State Department employees," Pompeo told reporters in Greece during a news conference with that country's foreign minister.

"We'll obviously do all the things that we are required to do by law," he said, without specifying how long it might take to comply.

Contributing: Maureen Groppe, Bart Jansen, Deirdre Shesgreen, Christal Hayes