Donald Trump is increasingly agitated by the House select committee investigating the Capitol attack, according to sources familiar with the matter, and appears anxious he might be implicated in the sprawling inquiry into the insurrection even as he protests his innocence.

The former president in recent weeks has complained more about the investigation, demanding why his former White House chief of staff, Mark Meadows, shared so much material about 6 January with the select committee, and why dozens of other aides have also cooperated.

Trump has also been perturbed by aides invoking the Fifth Amendment in depositions – it makes them look weak and complicit in a crime, he has told associates – and considers them foolish for not following the lead of his former strategist Steve Bannon in simply ignoring the subpoenas.

When Trump sees new developments in the Capitol attack investigation on television, he has started swearing about the negative coverage and bemoaned that the House minority leader, Kevin McCarthy, was too incompetent to put Republicans on the committee to defend him.

The former president’s anger largely mirrors the kind of expletives he once directed at the Russia inquiry and the special counsel investigation when he occupied the White House. But the rapidly accelerating investigation into whether Trump and top aides unlawfully conspired to stop the certification of Joe Biden’s victory at the 6 January joint session appears to be unnerving him deeply.

The portrait that emerges from interviews with multiple sources close to Trump, including current and former aides, suggest a former president unmoored and backed into a corner by the rapid escalation in intensity of the committee’s investigation.

A spokesperson for Trump did not respond to requests for comment.

But as Trump struggles to shield himself from the select committee, with public hearings next year and the justice department said to be tracking the investigation, the path ahead is only likely to be more treacherous.

The former president is especially attuned to his potential for legal exposure, even as he maintains he did nothing wrong in conferring about ways to overturn the 2020 election and encouraging supporters to march on the Capitol. He has expressed alarm to associates about repeated defeats in court as he seeks to stop the select committee obtaining some of the most sensitive of White House documents about 6 January from the National Archives, on grounds of executive privilege.

The reality is that with each passing day, the committee seems to be gathering new evidence about Trump’s culpability around the Capitol attack that might culminate with recommendations for new election laws – but also for prosecutions.

“I think that the justice department will keep a keen eye on what evidence the committee has accumulated, as well as looking out for witnesses for a potential case,” said Ryan Goodman, a former special counsel at the Department of Defense now a law professor at New York University.

“One of the outcomes of the committee’s work and the public hearings will be to demonstrate individuals who might be wanting to come forward as witnesses and that’s got to be very important to justice department prosecutors,” Goodman said.

House investigators are expected to soon surpass more than 300 interviews with Trump administration officials and Trump political operatives as part of a process that has yielded 30,000 documents and 250 tips via the select committee’s tip line.

The flurry of recent revelations – such as the disclosure of Meadows’s connection to a powerpoint outlining how Trump could stage a coup, as first reported by the Guardian – raises the specter that the select committee is swiftly heading towards an incriminating conclusion.

Trump’s associates insist they are not worried, at least for the moment, since the select committee has yet to obtain materials covered by executive privilege either through Meadows or the National Archives that could ensnare Trump personally.

The former president’s defenders are correct in that respect – the committee does not have messages that show Trump directing an attack on the Capitol, one source said – and Trump has vowed to appeal the National Archives case to the supreme court.

But no one outside the select committee, which is quietly making progress from a glass office on Capitol Hill with boarded-up windows and electronically secured doors, knows exactly what it has uncovered and whether the inquiry ends with a criminal referral.

The material Meadows turned over alone depicts an alarming strategy to stop Biden’s certification on 6 January, involving nearly the entire federal government and lieutenants operating from the Willard hotel in Washington.

One member on the select committee described the events around 6 January as showing a coalescence of multiple strategies: “There was a DoJ strategy, a state legislative strategy, a state election official strategy, the vice-president strategy. And there was the insurrection strategy.”

The text messages Meadows received on his personal phone implicate Trump’s eldest son, Don Jr, and Republican members of Congress. Texts Meadows turned over to the committee might also be used by an enterprising prosecutor as evidence of criminal obstruction to stop a congressional proceeding if the White House knew election fraud claims to be lies but still used them to stop Biden’s certification.

While Meadows never testified about the communications, a cadre of top Trump officials, from former acting national security adviser Keith Kellogg to Pence’s former chief of staff Marc Short, have moved to cooperate with House investigators.

The trouble for Trump – and part of the source of his frustration, the sources said – is his inability, out of office, to wield the far-reaching power of the executive branch to affect the course of the inquiry.

The limited success of strategies he hoped would stymie the committee – ordering aides to defy subpoenas or launching legal challenges to slow-walk the release White House records – has been jarring for Trump.

“I think what he’s finding is that as the ex-president, he has a lot less authority than he did as president. But his playbook doesn’t work if he’s not president,” said Daniel Goldman, former lead counsel in the first House impeachment inquiry into Trump.

In a reflection of dwindling legal avenues available to undercut the investigation, Trump has returned to launching attacks-by-emailed-statement on the select committee, stewing over his predicament and what he considers an investigation designed only to hurt him politically.

“The Unselect Committee itself is Rigged, stacked with Never Trumpers, Republican enemies, and two disgraced RINOs, Cheney and Kinzinger, who couldn’t get elected ‘dog catcher’ in their districts,” Trump vented last month.

In private, Trump is said to have reserved the brunt of his scorn for Meadows, furious with his former White House chief of staff for sharing sensitive communications on top of all the unflattering details about Trump included in his book this month.

Trump’s associates, however, have focused more on questioning the legitimacy of the select committee and its composition, arguing the fact that the House Speaker, Nancy Pelosi, appointed both Republican members reduces the investigation to a partisan political endeavor.

They also argue that none of the revelations to date – like the Guardian’s reporting on Trump’s call to the Willard hotel, during which he pressed operatives to stop Biden’s certification from taking place entirely – amounts to criminal wrongdoing.

But in the meantime, Trump is left with little choice but to wait for the committee’s report.

“The justice department seems to be more reactive than proactive,” Goodman said. “They might be waiting for the committee to wrap up its work to make criminal referrals.”