Former Vice President Mike Pence testified Thursday before a federal grand jury investigating former President Donald Trump’s effort to subvert the 2020 election, according to a source familiar with the matter.

Pence’s closed-door appearance marks an extraordinary flashpoint in special counsel Jack Smith’s probe. Smith’s team is investigating Trump’s last-ditch bid to pressure Pence into single-handedly derailing the transfer of power from Trump to Joe Biden on Jan. 6, 2021.

Pence’s appearance was accompanied by a frenzy of security activity at the federal courthouse in Washington, D.C., where U.S. Marshals and other security personnel conducted sweeps of the building before Pence was whisked inside without being spotted by cameras at the courthouse’s main entrances.

His arrival occurred just hours after a federal appeals court rejected Trump’s emergency bid to block Pence from testifying or limit the scope of prosecutors’ potential questions.