The House select committee investigating the deadly attack on the US Capitol on 6 January is poised to issue subpoenas to top Trump lieutenants involved in attempting to subvert the 2020 election results from a “command center” at the Willard hotel in Washington, according to a source familiar with the matter.

The subpoenas, which could be issued as soon as next week, reflect the select committee’s interest in events at the hotel just across from the White House, where Donald Trump’s most loyal aides plotted to keep him in office.

The select committee is targeting about 20 individuals connected to the Trump command center at the Willard, among them the legal scholar John Eastman, who outlined ways to deny Joe Biden the presidency, the source said.

The subpoenas seeking documents and testimony are aimed at obtaining the legal advice offered to Trump on how he could manipulate events on 6 January to stop certification of Biden’s election win, the source said.

House investigators are moving to pursue Trump lieutenants who gathered at the Willard to uncover the “centers of gravity” from which Trump and his advisers conspired, the source said – and whether the former president had advance knowledge of the Capitol attack.

The select committee appears to be seeking a full account of what transpired in several suites at the Willard in the days leading up to 6 January and during a final “war room” meeting the night before the Capitol attack.

The select committee is targeting Eastman after it emerged that he outlined scenarios for overturning the election in a memorandum presented at a White House meeting on 4 January with Trump, former vice-president Mike Pence and Trump’s chief of staff, Mark Meadows.

At that meeting, according to a source close to Trump, Eastman ran through the memo that detailed how at the joint session of Congress on 6 January Pence might refuse to certify electoral slates for Biden and thereby hand Trump a second term.

The former president seized on Eastman’s memo and relentlessly pressured Pence in the following days to use it to in effect commandeer the largely ceremonial electoral counting process, the Trump source said.

Trump was not successful in convincing Pence to reject Biden’s election win – an outcome top Trump aides blamed on the then vice-president’s chief of staff, Marc Short, the Trump source said – and Congress certified Biden as president after the Capitol attack.

Eastman has distanced himself from the memo, telling the Guardian the scenarios he outlined were not intended as advice. He also told the National Review he wrote the memo at the request of “somebody in the legal team” who he could not recall.

But Eastman appears to be a witness of importance, given he regularly attended meetings at the Willard with Giuliani and former Trump chief strategist Steve Bannon, acting effectively as a liaison with the White House, the Trump source said.

Eastman undermined his attempts to distance himself from the memo last week after he told the activist reporter Lauren Windsor that Pence declined to overturn the election, even though he had provided the legal reasoning, because “Pence is an establishment guy at the end of the day”.

The select committee is also considering a subpoena for Giuliani, the source said, since the former New York mayor and Trump lawyer led the legal effort from the Willard that involved trying to find and publicize allegations of electoral fraud.

Giuliani pressured state legislatures to challenge Biden victories and, even as the Capitol attack unfolded, cajoled Republican members of Congress to object to states’ electoral college votes, the Trump source said.

A spokesperson for the House select committee declined to comment about the targets or scope of forthcoming subpoenas. Neither Eastman nor a lawyer for Giuliani immediately responded to requests for comment.

The new line of inquiry centered on the Willard comes after the chairman of the select committee, Bennie Thompson, last week told reporters that he intended to subpoena Eastman, before later revealing that he had signed about 20 subpoenas.

Thompson said on Friday that he had hoped to issue the subpoenas to Eastman and other Trump lieutenants, but the timeline slipped as Democrats became consumed with crisis talks before the House passed Biden’s $1tn infrastructure package shortly before midnight.

The select committee remains in the evidence-gathering phase of its inquiry and has conducted interviews and depositions with more than 150 witnesses, according to the vice-chair of the panel, Liz Cheney.

“It is a range of engagements – some formal interviews, some depositions,” the Wyoming Republican said. “There really is a huge amount of work under way that is leading to real progress for us.”

Several former Trump officials, including Bannon, have resisted subpoenas. A former justice department official, Jeffrey Clark, on Friday refused to answer questions at a deposition, citing attorney-client privilege.

In a statement, Thompson raised the possibility of holding Clark in contempt of Congress. “Mr Clark’s complete failure to cooperate today is unacceptable,” he said. “We are willing to take strong measures to hold him accountable to meet his obligation.”