WASHINGTON — To circumvent White House efforts to impede the House impeachment inquiry, Democrats have launched a fresh offensive to bury the White House with numerous subpoenas and interview requests.

Democrats on Thursday issued a subpoena to Energy Secretary Rick Perry to produce documents related to President Donald Trump and the Ukraine. Perry led the U.S. delegation in May to the inauguration of Ukraine's president whose phone conversation with Trump is at the center of the House impeachment inquiry.

The lawmakers also issued a subpoena for testimony and documents to two associates of Giuliani who helped him in Ukraine, Igor Fruman and Lev Parnas. They were requested to appear for depositions Thursday and Friday before Congress, but they were arrested Wednesday night and face charges tied to campaign finance violations.

Since House Speaker Nancy Pelosi, D-Calif., formally announced the impeachment inquiry last month, Democrats have so far issued eight subpoenas and requested documents from Vice President Mike Pence.

Others who have been subpoenaed include Secretary of State Mike Pompeo; President Donald Trump’s personal lawyer Rudy Giuliani; White House chief of staff Mick Mulvaney; Defense Secretary Mark Esper; Russell Vought, the acting director of the Office of Management and Budget; and U.S. ambassador to the European Union Gordon Sondland.

A number of these officials had failed to comply with previous requests from Congress to either appear for testimony before lawmakers or share relevant documents and communications.

In an attempt to further block Democrats from seeking information, the White House said Tuesday that it would not turn over internal documents about Ukraine, arguing that they won’t comply with what they consider to be an invalid investigation.

Democrats plan, as a result of the White House’s stonewalling, to deploy a new strategy by requesting interviews with former administration officials whom they believe have pertinent information. Some of the requests will not be announced publicly, the sources said.

Committee chairmen plan to target these former officials because the administration has already tried to silence people who still work under it. Sondland, for example, did not show up for the deposition that was scheduled Tuesday for him to deliver closed-door testimony after the State Department directed him not to speak to Congress.

Former U.S. ambassador to Ukraine Marie Yovanovitch is scheduled to testify behind closed doors for her deposition Friday, but because she still works for the State Department, it's possible she may not show up either. Two congressional sources involved with the impeachment inquiry told NBC News that, as of now, they are still expecting Yovanovitch to appear for her deposition before the House Intelligence, Oversight and Foreign Affairs Committees.

Following Sondland’s absence, Democrats sent him a subpoena that ordered him to appear for a deposition Oct. 16.

Democrats believe that former administration officials, however, who are now private citizens, would not fall under the White House claims of executive privilege. Kurt Volker, who recently resigned as U.S. special envoy to Ukraine, testified before Congress last week, several days after he left the administration.

On Monday, Trump’s former Russia adviser, Fiona Hill, agreed to meet in a closed-door session with several congressional committees. A letter addressed to Hill, obtained by NBC, requested that she turn over several documents that date back to January 2017.

Congress returns next week from a two-week recess and Democrats are expected to speed up their investigation that could lead them to filing articles of impeachment against the president. Two sources told NBC News that Pelosi will hold another caucuswide conference call Friday afternoon so rank and file members can receive an update about the impeachment inquiry.