The US justice department’s criminal investigation into the January 6 Capitol attack collided with the parallel congressional investigation, causing federal prosecutors to seek a delay in proceedings in the seditious conspiracy case against the far-right Proud Boys group.

The two January 6 inquiries had largely managed to steer clear of each other even as both the justice department and a House select committee pursued the same ground. But it all came to head on Wednesday.

At a hearing in federal court in Washington, federal prosecutors and defendants in justice department’s seditious conspiracy case asked a federal judge to delay the August trial date of former Proud Boys national chairman Henry Tarrio, AKA Enrique Tarrio and other top members of the far-right group.

“It is reasonably foreseeable that information relevant to the defendants’ guilt (or innocence) could soon be released,” assistant US attorney Erik Kenerson wrote on Tuesday. “Inability to prepare their respective cases … is potentially prejudicial – to all parties.”

The request was granted “reluctantly” by US district judge Timothy Kelly, who said the trial will now start in December, agreeing that the select committee’s report and witness transcripts that are slated to be made public in September could upend preparations.

The justice department has run into the issue that because it is conducting a criminal investigation, its federal prosecutors are bound by strict rules requiring high standards of proof before they start issuing subpoenas and collecting evidence.

By contrast, the select committee, in conducting a congressional investigation examining the circumstances surrounding the Capitol attack, can issue subpoenas for documents and testimony whenever and however it likes, given the approval of a majority of its members.

That has meant the panel has amassed more than 1,000 transcripts of closed-door depositions conducted with key witnesses related to the January 6 inquiries, some of which the justice department believes are relevant to its cases but the panel had declined to share.

In a letter last week, Matthew Graves, the US attorney for the District of Columbia, and assistant attorneys general Kenneth Polite and Matthew Olsen complained their inability to access transcripts was hampering criminal investigations, including in the Proud Boys case.

“The select committee’s failure to grant the department access to these transcripts complicates the department’s ability to investigate and prosecute those who engaged in criminal conduct in relation to the January 6 attack on the Capitol,” they wrote in the letter.

The select committee relented and suggested it would not even wait until September but start making transcripts public as early as July. But lawyers for the Proud Boys took issue with both dates, saying the contents of the transcripts could bias a jury ahead of trial.

Not all of the defendants sought a delay. Tarrio opposed the request because “an impartial jury will never be achieved in Washington DC whether the trial is in August, December, or next year”. Ethan Nordean, another prominent Proud Boys figure, opposed the request unless he was freed from pre-trial detention.

The potential for the transcripts to influence a jury pool has been a recurring complaint for the Proud Boys lawyers, who argue the January 6 hearings – which started three days after Tarrio and others were charged with seditious conspiracy – will irreparably taint a jury.

Federal prosecutors have pushed back, contending that people in Washington were no more likely to have seen the hearings than people in New York or Miami. Still, the government agreed for the need for breathing space between the trial and transcripts being made public.

The justice department, meanwhile, has its own concerns with the transcripts’ release and would seemingly prefer to get the transcripts in private to compare what witnesses have told the select committee and what they have secretly told a grand jury.

At least two members of the Proud Boys have testified before the select committee in closed-door depositions: Tarrio, who has been charged with seditious conspiracy and other crimes, and Jeremy Bertino, who has been mentioned in court filings but is currently not charged.

Also on Wednesday, the justice department issued new subpoenas to at least three people connected to the Trump campaign’s potentially illegal plan to send fraudulent election certificates to Congress as part of the effort to overturn the 2020 election results.

The confirmed recipients of the grand jury subpoenas were Brad Carver, a Georgia Republican Party official who was a Trump elector, Thomas Lane, a Trump campaign official in Arizona and New Mexico, and Sean Flynn, a Trump campaign aide in Michigan, the New York Times reported.