Former Trump lawyer John Eastman, who was connected to efforts to stop the certification of Joe Biden’s presidential election win on 6 January, will plead the fifth amendment protection against self-incrimination before the House select committee investigating the Capitol attack.

The move by Eastman, communicated in a letter to the select committee by his attorney, is an extraordinary step and appears to suggest a growing fear among some of Trump’s closest advisers that their testimony may implicate them in potential criminality.

“Dr Eastman has a more than reasonable fear that any statements he makes pursuant to this subpoena will be used in an attempt to mount a criminal investigation against him,” Eastman’s lawyer, Charles Burnham, told the select committee in a letter on Wednesday.

The select committee issued a subpoena to Eastman last month as they sought to uncover the extent of his role in Trump’s scheme to prevent Biden from being certified as president and return himself to office for a second term despite losing the 2020 election.

House investigators also took an interest in Eastman after it emerged that he played an integral part in a 4 January Oval Office meeting where he presented a memo advising then vice-president Mike Pence about ways to stop and delay Biden’s certification from taking place.

But in responding to the subpoena, Eastman’s attorney told the select committee that the former Trump lawyer would assert the fifth amendment to protect himself from the rapidly expanding investigation that has so far ensnared dozens of top Trump allies.

The letter made mostly procedural objections to the select committee’s inquiry, complaining that the panel only has members appointed by the House speaker, Nancy Pelosi, conducts its depositions behind closed doors, and that the scope of the subpoena was excessively broad.

Neither Eastman nor Burnham immediately responded to a request for comment on Friday.

The decision by Eastman to protect himself from self-incrimination came a day after the Guardian revealed that Eastman was connected to a phone call that Trump placed hours before the Capitol attack, seeking ways to somehow stop Biden’s certification on 6 January.

According to multiple sources familiar with the call, Trump, on at least one call placed from the White House, pressed his lieutenants at the Willard – led by his lawyer Rudy Giuliani and Eastman – about ways to prevent Biden from being pronounced president.

The move by Eastman also makes him the second ally of the former president to claim the fifth amendment with the select committee, after the former Trump DoJ official Jeffrey Clark announced that he would invoke the protection in a deposition scheduled for Saturday.