Top Republicans under scrutiny for their role in the events of 6 January have embarked on a campaign of threats and intimidation to thwart a Democratic-controlled congressional panel that is scrutinizing the Capitol attack and opening an expanded investigation into Donald Trump.

The chairman of the House select committee into the violent assault on the Capitol, Bennie Thompson, in recent days demanded an array of Trump executive branch records related to the insurrection, as members and counsel prepared to examine what Trump knew of efforts to stop the certification of Joe Biden’s election win.

House select committee investigators then asked a slew of technology companies to preserve the social media records of hundreds of people connected to the Capitol attack, including far-right House Republicans who sought to overturn the results of the 2020 election.

The select committee said that its investigators were merely “gathering facts, not alleging wrongdoing by any individual” as they pursued the records in what amounted to the most aggressive moves taken by the panel since it launched proceedings in July.

But the twin actions, which threatened to open a full accounting of Trump’s moves in the days and weeks before the joint session of Congress on 6 January, has unnerved top House Republicans, according to a source familiar with the matter.

The House minority leader, Kevin McCarthy, decried the select committee’s investigation as a partisan exercise and threatened to retaliate against any telecommunications company that complied with the records requests.

“A Republican majority will not forget,” he warned, in remarks that seemed to imply some future threat against the sector.

The warning from the top Republican in the House amounted to a serious escalation as he seeks to undermine a forensic examination of the attack perpetrated by Trump supporters and domestic violent extremists that left five dead and nearly 140 injured.

But his remarks – which members on the select committee privately consider to be at best, harassment, and at worst, obstruction of justice – reflects McCarthy’s realization that he could himself be in the crosshairs of the committee, the source said.

Most of McCarthy’s efforts to undercut the inquiry to date, such as sinking the prospects of a 9/11-style commission to scrutinize the Capitol attack, have been aimed at shielding Trump and his party from what the select committee might uncover.

But deeply alarmed at the efforts by House select committee investigators to secure his personal communications records for the fraught moments leading up to and during the Capitol attack, McCarthy went on the offensive to pre-emptively protect himself, the source said.

McCarthy was among several House Republicans who desperately begged Trump to call off the rioters as they stormed the Capitol in his name, only to be rebuffed by Trump, who questioned why McCarthy wasn’t doing more to overturn the election.

Thompson previously told the Guardian in an interview that such conversations with Trump would be investigated by the select committee, raising the prospect that McCarthy could be forced to testify about what Trump appeared to be thinking and doing on 6 January.

The statement from McCarthy asserted, without citing any law, that it would be illegal for the technology companies to comply with the records requests – even though congressional investigators have obtained phone and communications records in the past.

The threat is unlikely to be viewed as a violation of federal witness tampering law, which, as part of a broader obstruction of justice statute, makes it a felony under some circumstances to try to dissuade or hinder cooperation with an official proceeding.

Congressman Jamie Raskin, a member of the select committee and the former lead impeachment manager in Trump’s second trial, said that he was appalled by McCarthy’s remarks, which he described as tantamount to obstruction of justice.

“He is leveling threats against people cooperating with a congressional investigation,” Raskin said. “Why would the minority leader of the House of Representatives not be interested in our ability to get all of the facts in relation to the January 6th attack?”

Meanwhile, other members on the select committee have also seized on McCarthy’s threat as a reminder that Republicans could not be trusted to engage in the inquiry in good faith, according to a source connected to the 6 January investigation.

It also underscored to them, the source said, the nervousness among top Republicans as the select committee ramps up its work, even though the inquiry is still in its early days and has yet to sift through thousands of pages of expected evidence.

Emboldened by McCarthy’s combative stance, Trump denounced the select committee as a “partisan sham”, while Republicans under scrutiny by the panel such as Marjorie Taylor Greene threatened any companies that complied with the records requests would be “shut down”.

The chairman of the House Freedom Caucus, Andy Biggs, is now also asking McCarthy to remove from the Republican conference Liz Cheney and Adam Kinzinger – the two vocal critics of Trump appointed to the select committee – whom he called “spies” for Democrats.

Biggs on Thursday suggested in a letter, first reported by CNN, that Cheney and Kinzinger should be ejected because they are involved in investigating Republicans over 6 January and the party should be able to strategize without having the pair present at conference meetings.

Still, McCarthy remains unable to shape an investigation likely to prove politically damaging to Trump and to Republicans at the ballot box at the midterms next year, a reality that has come largely as a result of his own strategic miscalculations.

The proposed 9/11-style commission into the Capitol attack had envisioned a panel with equal power between Democrats and Republicans, and McCarthy’s decision to boycott the select committee in a flash of anger inadvertently left Trump without any defenders.