Former Vice President Mike Pence has been subpoenaed by the special counsel overseeing investigations into efforts by former President Donald Trump and his allies to overturn the results of the 2020 election, according to a person with direct knowledge of the event.

The subpoena to Pence as part of the investigation by special counsel Jack Smith was served in recent days, according to the person, who spoke to The Associated Press on condition of anonymity Thursday to discuss a sensitive issue.

The extraordinary scenario of a former vice president potentially testifying against his former boss in a criminal investigation comes as Pence considers launching a 2024 Republican presidential bid against Trump. The two have been estranged since a mob of Trump’s supporters stormed the U.S. Capitol building on Jan. 6, 2021, in an attempt to stop Democrat Joe Biden’s victory.

The subpoena is an aggressive step from a prosecutor who for years led the Justice Department’s public corruption section and who oversaw indictments against major political figures. The move sets the stage for a likely executive privilege fight, given Pence’s close proximity to Trump for four years as major decisions were being contemplated and planned. It is unclear whether efforts to secure voluntary testimony from Pence stalled before the subpoena was issued.

Spokespeople for Pence and Smith declined to comment on the issuance of the subpoena, which was first reported by ABC News.

Pence was a central figure in Trump’s efforts to stay in power after losing the 2020 election. Trump falsely insisted that his vice president, who had a ceremonial role in overseeing the certification of the election, could simply reject the results and send them back to the battleground states he contested.

‘Hang Mike Pence’

On Jan. 6, Trump supporters, driven by the lie that the election was stolen, marched to the Capitol building, brutally pushed past the police and smashed through the windows and doors while Pence was presiding over the certification of Biden’s victory. The vice president was steered to safety with his staff and family as some in the mob chanted, “Hang Mike Pence!”

While the mob was in the Capitol, Trump tweeted, “Mike Pence didn’t have the courage to do what should have been done to protect our Country and our Constitution.”

Smith, who was named special counsel in November by Attorney General Merrick Garland, has been tasked with overseeing investigations into Trump’s attempts to subvert his defeat, his actions leading up to the Jan. 6 insurrection at the Capitol and his possession of top-secret government documents at his Florida estate, Mar-a-Lago.

Federal prosecutors have been especially focused on a scheme by Trump allies to elevate fake presidential electors in key battleground states won by Biden as a way to subvert the vote, issuing subpoenas to multiple state Republican party chairs.

Federal prosecutors have brought multiple Trump administration officials before the grand jury for questioning, including former Trump White House counsel Pat Cipollone and Pence’s own former chief of staff, Marc Short.

In a sign of the expanding nature of the investigation, election officials in multiple states whose results were disputed by Trump have received subpoenas asking for communications with or involving Trump and his campaign aides.

A House committee investigating the Jan. 6 attack recommended that the Justice Department bring criminal charges against Trump and associates who helped him launch a pressure campaign to try to overturn his 2020 election loss.