The US congressional committee investigating the Capitol attack has issued a blitz of subpoenas to some of Donald Trump’s top lawyers – including Rudy Giuliani – as it examines whether the former president oversaw a criminal conspiracy on 6 January 2021.

The House panel subpoenaed four of Trump’s legal team on Tuesday: the former president’s personal attorney Rudy Giuliani and his associate Boris Epshteyn, as well as Sidney Powell and Jenna Ellis, who all defended Trump’s baseless voter fraud claims as he attempted to overturn the election result.

Congressman Bennie Thompson, the chairman of the select committee, said in a statement that the panel issued the subpoenas to the four Trump lawyers because they were “in direct contact with the former president about attempts to stop the counting of electoral votes”.

The move by the select committee amounts to another dramatic escalation in the investigation, as the orders compel Trump’s lawyers to produce documents and testimony, suggesting the panel believes the lawyers may have acted unlawfully.

In its most aggressive move, the select committee ordered Giuliani to testify under oath about his communications with Trump and Republican members of Congress regarding strategies for delaying or overturning the election results.

Thompson said in the subpoena letter to Giuliani that House investigators also wanted to question him about his efforts to subvert Biden’s win, urging Trump to unlawfully seize voting machines and pressuring certain state legislators to decertify their results.

Epshteyn is the former communications director for Trump’s 2016 inauguration, who worked alongside Giuliani in the Willard hotel in the days before January 6 as Trump sought desperately to grant himself a second term.

Citing a report by the Guardian about how Trump pressed his lieutenants at the Willard hotel to prevent Biden’s certification hours before the Capitol attack, Thompson said in the subpoena letter to Epshteyn that the panel wanted to ask about his discussions with Trump.

The Guardian report revealed a direct line between the White House and the Trump “war room” at the Willard hotel, and showed that Trump personally pushed to stop Biden’s certification, which was also the purported aim of the Capitol attack.

The select committee noted Epshteyn was also close to the former president’s disinformation effort about widespread voter fraud, as he attended a Trump campaign press conference promoting lies about a stolen election.

House investigators have been mulling subpoenas to Giuliani and Ephsteyn for weeks, according to a source close to the inquiry. The fact that the panel moved ahead with the orders suggests they suspect criminality that could overcome claims of attorney-client privilege.

“The attorney-client privilege does not operate to shield participants in a crime from an investigation into a crime,” Congressman Jamie Raskin, a member of the select committee, said of a subpoena to Trump’s lawyers in an earlier interview with the Guardian.

Thompson, the chairman of the select committee, also suggested that its investigation into Giuliani and Epshteyn would focus on Trump’s calls to the Willard hotel, saying that the panel would scrutinize White House call detail records held by the National Archives.

In the subpoena letter to Powell, who is already sanctioned by a federal judge for misconduct relating to her lawsuits challenging Biden’s win, Thompson said the panel wanted the evidence she used to advance disinformation about the election.

He said the panel was also interested in her role as an external lawyer for the Trump campaign, which saw her urge Trump to seize voting machines around the country in an attempt to find evidence that foreign adversaries had hacked the machines and caused Trump’s defeat. No such evidence was found.

In the subpoena letter to Ellis, the select committee said it was interested in her efforts to overturn the election results and her two memos that erroneously said then-Vice President Mike Pence could reject or delay counting electoral votes for Biden on January 6.

The select committee gave the four lawyers until the start of February to produce documents requested by its investigators and appear for depositions scheduled later in the month. Giuliani, Powell and Ellis could not be reached for comment. Epshteyn declined to comment.