The attorney general, Merrick Garland, is facing more political pressure to move faster and expand the US Department of Justice’s investigation into the January 6 Capitol attack and charge Donald Trump and some of his former top aides.

With mounting evidence from the January 6 House panel, court rulings and news reports that Trump engaged in a criminal conspiracy in his aggressive drive to thwart Joe Biden’s election win in 2020, Garland and his staff face an almost unique decision: whether to charge a former US president.

Ex-justice officials caution, however, that while there’s growing evidence of criminal conduct by Trump to obstruct Congress from certifying Biden’s win on January 6 and defraud the government, building a strong case to prove Trump’s corrupt intent – a necessary element to convict him – probably requires more evidence and time.

In an important speech in January this year, Garland said he would hold “all January 6 perpetrators, at any level” accountable, if they were present at the Capitol that day or not, who were responsible for this “assault on our democracy”, which suggested to some ex-prosecutors that Trump and some allies were in his sights.

But rising pressures on Garland to move faster with a clearer focus on Trump and his top allies have come from Democrats on the House panel investigating the Capitol attack.

Those concerns were underscored this past week when the House sent a criminal referral to the justice department charging contempt of Congress by two Trump aides, trade adviser Peter Navarro and communications chief Dan Scavino, who refused to cooperate after being subpoenaed.

“We are upholding our responsibility, the Department of Justice must do the same,” panel member Adam Schiff said. Likewise, Congresswoman Elaine Luria urged Garland to “do your job so we can do ours.”

About four months ago, the House sent a criminal contempt of Congress referral to the justice department for the former White House chief of staff Mark Meadows, but so far he has not been indicted.

Some former top DoJ officials and prosecutors, however, say Garland is moving correctly and expeditiously in pursuing all criminal conduct to overturn Biden’s election in its sprawling January 6 inquiry.

“When people (including many lawyers) criticize the DoJ for not more clearly centering the January 6 investigation on Trump, they are expressing impatience rather than a clear understanding of the trajectory of the investigation,” the former justice inspector general Michael Bromwich told the Guardian.

“DoJ is methodically building the case from the bottom up. It is almost surely the most complex criminal investigation in the nation’s history, involving the most prosecutors, the most investigators, the most digital evidence – and the most defendants,” he added.

Bromwich added that “people view the scores of ongoing criminal prosecutions of participants in the January 6 insurrection as somehow separate from the investigation of Trump. They are not. He is the subject of the investigation at the top of the pyramid. People need to carefully watch what is happening, not react based on their impatience.”

The department’s investigation is the biggest one ever. More than 750 people have been charged so far with federal crimes, and about 250 have pleaded guilty.

Still, concerns about the pace of the investigation – and why charges have not been filed against Trump – have been spurred in part by a few revelations over the last couple of months.

Last month, for instance, federal judge David Carter in a crucial court ruling involving a central Trump legal adviser, John Eastman, stated that Trump “more likely than not” broke the law in his weeks-long drive to stop Biden from taking office.

“Dr Eastman and President Trump launched a campaign to overturn a democratic election, an action unprecedented in American history,” Carter wrote in a civil case which resulted in an order for Eastman to release more than 100 emails he had withheld from the House panel.

Similarly, the January 6 select committee made a 61-page court filing on 2 March that implicated Trump in a “criminal conspiracy” to block Congress from certifying Biden’s win.

On another legal front that could implicate Trump and some top allies, the deputy attorney general, Lisa Monaco, revealed in January that the DoJ was starting a criminal investigation into a sprawling scheme – reportedly spearheaded by Trump’s ex-lawyer Rudy Giuliani and Trump campaign aides – to replace legitimate electors for Biden with false ones pledged to Trump in seven states that Biden won.

Further, the Washington Post reported late last month that the DoJ had begun looking into the funding and organizing of the January 6 “Save America” rally in Washington involving some Trump allies. Trump repeated his false claims at the rally that the election was stolen.

“We won this election, and we won it by a landslide,” Trump falsely told the cheering crowd. “You don’t concede, when there’s theft involved,” he said, urging the large crowd to “fight like hell”, shortly before the Capitol attack by hundreds of his supporters that led to 140 injured police and several deaths.

A Trump spokesperson, Taylor Budowich, has called the House January 6 inquiry a “circus of partisanship”. And Budowich attacked Judge Carter’s ruling as “absurd and baseless”, noting that Carter was a “Clinton-appointed judge in California”.

Dennis Aftergut, a former federal prosecutor, told the Guardian that recent actions by the House January 6 panel and by the DoJ, along with court opinions, have notably increased legal threats to Trump. “Anyone would need ice in their veins not to feel the heat when all three branches of the federal government are breathing down your neck,” he said.

On the issue of whether Trump may be indicted, Donald Ayer, who served as deputy attorney general in the George HW Bush administration, said “the critical question should be whether there is adequate proof of wrongful intent.” Citing Carter’s ruling that Trump “more likely than not” broke the law, Ayer said that “the evidence of such intent has recently become a lot stronger.”

Nonetheless, Ayer and Aftergut stress Garland has to juggle competing priorities lest he politicize his department, while being extra careful to ensure any charges he may bring against Trump will stand up in court.

“Garland’s between the rock of defending one justice department ideal and the hard place of protecting another. On one hand, no person is above the law. On the other hand, the department needs to avoid, as much as possible consistent with the first ideal, appearing political,” Aftergut said.

“There’s nothing easy about the position Garland’s in,” Aftergut added. “The safest course, before considering a prosecution of a former president, would be to demand considerably more evidence of guilt than you’d require in any other case.”

Ayer added: “Garland is right not to be discussing the specifics of whether and how Trump may be indicted,” a stance Garland has adopted to protect the DoJ’s credibility as not political. At the same time, Ayer suggested that Garland “should spend more time talking to the country about impartial justice and the idea that no person is above the law”.

There are clear risks in moving too fast to appease critics.

“Garland must make his decisions based on the law in relation to the facts,” the former federal prosecutor Michael Zeldin said. “The more politicians endeavor to pressure Garland to act, it runs the risk that any decision Garland makes will be seen as politically motivated rather than based on purely legal considerations.”

That seems to fit with Garland’s approach. In his 5 January speech this year, Garland emphasized, “we follow the physical evidence. We follow the digital evidence. We follow the money. But most important, we follow the facts – not an agenda or an assumption. The facts tell us where to go next.”

And, if there is enough evidence, following the rules could end up with Trump getting charged.

“DoJ will never announce that it is investigating Trump and his inner circle. Such an announcement would violate DoJ policy to neither confirm nor deny the existence of an investigation,” said Barbara McQuade, a professor from practice at the University of Michigan Law School and a former attorney for the eastern district in Michigan.

Garland, McQuade added, “is avoiding the mistake FBI director Jim Comey made in investigating Hillary Clinton, for which Comey was properly criticized”, referring to two status reports about the investigation made in the months before the 2016 election.

Ultimately, McQuade said that Garland’s “biggest challenge will be proving that Trump had corrupt intent or intent to defraud, both of which would require proving that he knew his fraud claims were false. It can be very difficult to prove what was in someone’s mind, but it is not impossible.”