Originally Published: 26 JUL 22 13:48 ET

Updated: 26 JUL 22 15:31 ET

By Tierney Sneed and Evan Perez, CNN

(CNN) -- The move by the Justice Department to bring two top aides to former Vice President Mike Pence in front of a federal grand jury is the most aggressive public step taken yet by prosecutors investigating the plots to subvert the 2020 election.

It signals that the Department's probe has reached inside former President Donald Trump's White House and that investigators are looking at conduct directly related to Trump's and his closest allies' efforts to overturn his election defeat.

Former Pence chief of staff Marc Short testified in front of a grand jury in DC on Friday, CNN has confirmed. Fellow Pence aide Greg Jacob testified last week as well, according to The New York Times and The Wall Street Journal.

Short and Jacob both were present in key meetings in the lead-up to the January 6 insurrection that were part of a pressure campaign to convince Pence to disrupt Congress' certification of President Joe Biden's electoral win.

Jacob, a former legal adviser to Pence, has participated in the House January 6 select committee investigation, even testifying publicly at a hearing last month. (He has not responded to CNN's inquiries about reports of his grand jury testimony.)

Short confirmed to CNN's Erin Burnett Monday night he complied with a grand jury subpoena but declined to discuss any details about his appearance. Short previously sat for a deposition in the House investigation.

Short's and Jacob's accounts were among the evidence that a federal judge in California cited in concluding that Trump and his allies may have been planning a crime in their plot to disrupt the transfer of presidential power.

How to approach Trump in the January 6 investigation has been a delicate question for the Justice Department. The department has faced immense pressure from lawmakers, former prosecutors and others to focus on the ringleaders of the 2020 election reversal gambits. But any investigation of a former president raises a host of highly sensitive and potentially explosive political and legal questions.

That Pence's inner circle is now being compelled to cooperate in the probe suggests that at least some of those obstacles have been cleared.

Last week, Attorney General Merrick Garland was visibly frustrated when a reporter grilled him about the possibility that the former president would be charged, as the former judge repeated vague assertions that, "No person is above the law in this country."

In an interview with NBC on Tuesday, Garland said DOJ would look at any interference on the "lawful transfer of power."

"I will say again that we will hold accountable anyone who is criminally responsible for attempting to interfere with the legitimate, lawful transfer of power from one administration to the next," Garland said.

Rep. Adam Kinzinger, one of the two Republicans on the House select committee, said the Pence aides testifying is a "positive" development.

"The Department of Justice needs to look at any possible criminality," Kinzinger said Tuesday on CNN's "New Day." "I ask, what have they been doing for the last year and a half. That said, if they are moving forward on looking at this stuff, that's positive for the country."

Short, Kinzinger added, is a good source.

"He certainly knows a lot of what happened, and I think under oath you can be pretty much committed that he'll tell the truth," the congressman said.

Witnesses to inflection points in the pressure campaign on Pence

The Justice Department declined to comment on the grand jury subpoenas of Short and Jacob. But scrutiny elsewhere of Trump's election subversion gambits highlight the relevant events that the two aides witnessed.

According to the House select committee's public presentations, Jacob was present at a January 4, 2021, Oval Office meeting with Trump and his attorney John Eastman during which Eastman discussed a plan to have Pence not count the Biden electors that would be put forward at the January 6 certification ceremony.

Short attended that meeting as well, and another meeting with Eastman and Jacob on January 5, where Eastman's push to have Pence disrupt the certification was further discussed for another two and a half hours.

"What most surprised me about that meeting was that when Mr. Eastman came in he said, 'I'm here to request that you reject the electors,'" Jacob said at a House committee hearing in June, while referring to a proposal more aggressive than what Eastman had been recommended in the January 4 meeting. Jacob also took notes memorializing Eastman's assertion, according to the committee.

Jacob recalled Eastman acknowledging at one of those meetings that such a maneuver would violate the Electoral Count Act, with Jacob testifying that Eastman thought the law was unconstitutional. Jacob also described a conversation with Eastman where Eastman conceded that no justice on the Supreme Court would support his position.

Jacob's account of the meetings, and emails he exchanged with Eastman, were also presented earlier this year to a federal judge who was considering whether to order Eastman to produce certain emails to the committee.

In approving the release of the documents, US District Judge David O. Carter indicated that he thought Trump's conduct may have been criminal, concluding that it was "more likely than not that President Trump corruptly attempted to obstruct the Joint Session of Congress on January 6, 2021."

"The illegality of the plan was obvious," Carter wrote. "Our nation was founded on the peaceful transition of power, epitomized by George Washington laying down his sword to make way for democratic elections. Ignoring this history, President Trump vigorously campaigned for the Vice President to single-handedly determine the results of the 2020 election."

Other signs that the DOJ investigation has escalated

While Jacob and Short's grand jury appearances were the most significant public development in the Justice Department's probe, there have been other major signs that the department was moving beyond the low-hanging fruit of prosecuting rioters who physically breached the Capitol on the day of the certification.

Federal investigators seized electronics from Eastman in late June, and also raided the home of former Justice Department official Jeffrey Clark, another prominent player in Trump's plots.

The department has issued numerous subpoenas in recent weeks seeking information in all seven battleground states where Trump's campaign assembled false electors as part of a bid to subvert the Electoral College. Among those subpoenaed were GOP officials in Arizona, Georgia, Michigan and Pennsylvania, CNN previously reported.

The House committee has described the fake electoral plot, in which Trump supporters would be fraudulently put forward as substitutes for the legitimate Biden electors, as part of the broader push to have Pence interrupt the certification of Biden's win. Select Committee Chairman Bennie Thompson has also indicated that it's working to share information with the Justice Department from that aspect of the lawmakers' probe.

Meanwhile, the Justice Department has also upped the ante in its prosecutions of those who violently assaulted the Capitol, with seditious conspiracy charges brought against prominent members of fringe groups accused of planning to "oppose by force the lawful transfer of presidential power."

The DOJ's latest moves involving Pence aides come as Pence has broken with Trump by support Republican politicians competing in GOP primaries against Trump's favored, including in races in Georgia and Arizona where Trump's lies about the 2020 election have featured prominently.