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.