Pat Cipollone, the White House counsel during the Trump administration, appeared at federal court on Friday to testify before a grand jury probing events surrounding the Jan. 6, 2021, attack on the U.S. Capitol.

Cipollone and his attorney Michael Purpura were greeted in the hallway by Thomas Windom, the lead prosecutor investigating a failed bid by former President Donald Trump's allies to overturn the results of the 2020 election by submitting alternative slates of fake electors to the U.S. National Archives.

They proceeded to the third floor, where the grand jury meets, according to a Reuters witness.

Former White House Deputy Counsel Pat Philbin also was expected at the federal courthouse in Washington later on Friday. He and Cipollone were subpoenaed earlier this year to appear as witnesses.

The two men are the two most high-profile witnesses to date to appear before the grand jury. Others who have appeared to testify include former Vice President Mike Pence's chief of staff, Marc Short, and Greg Jacob, who was Pence's top counsel.

The grand jury, which convenes each Friday in the federal courthouse in Washington, is known to be specifically probing the fake electors plot.

Electors are people chosen to formally cast a state's electoral votes in the U.S. Electoral College system used in presidential elections.

The fake elector plot has featured prominently in multiple hearings of the Democratic-led House of Representatives committee probing the attack on the U.S. Capitol.

Rusty Bowers, the Arizona state House Republican speaker, told the panel that Trump and his close aides, including his personal lawyer Rudy Giuliani and adviser John Eastman, urged him to reject the election results. Bowers refused their request.

The Justice Department has since seized Eastman's phone and searched its contents, as part of the ongoing probe.

It has also seized the phones of Republican congressman Scott Perry, a Trump ally, and Jeffrey Clark, a former top Justice Department lawyer who also tried to promote a plan which entailed urging Georgia state lawmakers to convene a new session and submit alternate slates of electors on the false premise of voter fraud.

In recent months, the Justice Department has delivered grand jury subpoenas to numerous individuals who may have knowledge about the bid to submit the phony slates, as well as some of the individuals who signed the bogus certificates themselves.

The subpoenas ask for copies of documents related to "any effort, plan or attempt to serve as an elector in favor of Donald J. Trump and/or Mike R. Pence."

They are also seeking copies of communications between would-be electors and any federal government employees or any employees or agents of Trump, as well as communications with a long list of people including Giuliani, who promoted Trump's bogus claims of election fraud, and Eastman.