House Democrats' impeachment inquiry has quickly expanded since Speaker Nancy Pelosi signed off last week, with committee leaders already sending or threatening subpoenas to key Trump administration officials involved in the Ukraine controversy, and scheduling testimony for individuals who could bolster their case.

House Intelligence Committee Chairman Adam Schiff, D-Calif., who is playing a central role in leading the impeachment inquiry, has vowed to move quickly through this process, identifying individuals the committee wants to hear from.

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“We will move as expeditiously as possible,” Schiff told CNN last week. “But we have to see what witnesses are going to make themselves available and what witnesses are going to require compulsion.”

Over the weekend, Schiff, House Foreign Affairs Committee Chairman Eliot Engel, D-N.Y., and House Oversight Committee Chairman Elijah Cummings, D-MD., subpoenaed Secretary of State Mike Pompeo for documents related to Ukraine they said were critical to their probe.

“Your failure or refusal to comply with the subpoena shall constitute evidence of obstruction on the House’s impeachment inquiry,” they wrote.

Schiff has also scheduled closed-door testimony from intelligence community Inspector General Michael Atkinson, who is slated to appear before the panel on Friday. Atkinson is the intelligence community watchdog who first received the whistleblower complaint and transmitted it to Acting Director of National Intelligence Joseph Maguire.

Schiff has also scheduled depositions for several State Department officials later next month.

In their demands to the State Department, Schiff and the committee chairs asked for correspondence and documents dating back to Jan. 20, 2017. The documents relate to revelations that Trump pressed Ukrainian officials to investigate former Vice President Joe Biden and his son Hunter, as well as any Biden business activity in Ukraine. They also demanded a list of State Department officials who may have been involved in Trump’s July 25 phone call with Ukrainian President Volodymyr Zelensky, as well as State Department records about former New York City mayor and Trump personal attorney Rudy Giuliani and records relating to U.S. military aid to Ukraine.

Further, the chairs notified Pompeo that they have scheduled depositions for State Department officials slated for next month. The depositions are for former Ambassador to Ukraine Marie Yovanovitch, former U.S. Special Representative for Ukraine Kurt Volker—who resigned from his post last Friday—Deputy Assistant Secretary George Kent, Counselor T. Ulrich Brechbuhl, and U.S. Ambassador to the European Union Gordon Sondland.

All officials, except Kent, were named in the whistleblower complaint accusing Trump of “using the power of his office to solicit interference from a foreign country in the 2020 U.S. election.” The whistleblower alleged that Trump pressured the Ukrainian president into investigating the Bidens, and Democrats have claimed this appeared linked to U.S. military aid to Ukraine -- which Trump denies.

Schiff and the chairs warned that “the failure of these department employees to appear for their scheduled depositions shall constitute evidence of obstruction of the House’s impeachment inquiry.”

Schiff hinted last week that any stonewalling from the White House would simply “strengthen” their case on obstruction—which was a focus of informal House impeachment investigations prior to the Ukraine controversy.

Despite the rapidly surfacing developments, Republicans are casting the impeachment inquiry as Russia investigation 2.0.

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“The Democrats learned from the Russia hoax that it’s a mistake to wait for the facts to come out if the facts may not be on their side,” a House GOP source told Fox News. “So the whole point now is to gin up a media frenzy and proceed lightning fast to impeachment based mostly on emotion and hysteria.”

Trump tweeted Monday: "The Fake Whistleblower complaint is not holding up. It is mostly about the call to the Ukrainian President which, in the name of transparency, I immediately released to Congress & the public. The Whistleblower knew almost nothing, its 2ND HAND description of the call is a fraud!"

Meanwhile, Schiff has said he plans to subpoena Giuliani.

“We’re going to need evidence from Rudy Giuliani,” Schiff told “60 Minutes.” “And it’s our intention, as soon as first thing next week, to subpoena him for documents. And there may very well come a time where we want to hear from him directly.”

Giuliani played a key role in seeking political dirt on the Bidens from Ukrainian officials. The controversial July 25 phone call between Trump and Zelensky revealed that the president wanted to get Zelensky on the phone with Giuliani, as well as Attorney General Bill Barr, to discuss a potential investigation.

Barr, who has sought to distance himself from the controversy, has not been subpoenaed, and it is unclear whether the committees will attempt to compel his testimony.

Democrats, though, plan to continue working expeditiously on the inquiry, with some reports indicating they could even be prepared to introduce formal articles of impeachment against Trump later this fall.

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Pelosi, D-Calif., though, has said their time table is fluid, and that she has “no idea” how long the inquiry will take.

“It will take as long as the intelligence committee follows the facts and when they are ready,” she said.

Pelosi announced the formal inquiry last week, amid the chaos surrounding Trump's controversial phone call with the Ukrainian president. But prior to announcing the formal impeachment inquiry, House Democrats already were running informal investigations in an effort to impeach the president. The House Judiciary Committee was probing obstruction of justice and alleged misconduct by the president.

The formal impeachment inquiry takes the House Judiciary Committee's investigation and combines it with other Trump-focused probes being run out of the House Committees on Intelligence, Foreign Affairs, Financial Services, Oversight and Ways and Means.

But despite new polls showing an increase in support for the formal impeachment inquiry from voters, the president is blasting it as “another witch hunt.”

“The Greatest Witch Hunt in the history of our Country!” Trump tweeted Monday.

Fox News' Melissa Leon and The Associated Press contributed to this report.