A former Trump campaign aide appeared for hours before a federal grand jury Friday, after he defiantly insisted in a series of news interviews just days earlier that he intended to defy a subpoena in special counsel Robert Mueller's Russia investigation.

Sam Nunberg spent more than six hours inside the federal courthouse in Washington. He declined to speak with journalists on the way in or out of the building, and it was not immediately clear what testimony he offered to the grand jury or what documents he provided.

His appearance marked a turnabout from extraordinary public statements Monday when Nunberg, in multiple interviews, balked at complying with a subpoena that sought his appearance before the grand jury as well as correspondence with other campaign officials. In doing so, he became the first witness in the Mueller probe to openly threaten to defy a subpoena.

Nunberg said he worked for hours to produce the thousands of emails and other communications requested by Mueller, who is investigating whether Donald Trump's campaign improperly coordinated with Russia during the 2016 presidential election.

Trump has denied any wrongdoing. His lawyers are currently negotiating the terms and scope of a possible interview with Mueller's office.