The Senate Intelligence Committee on Wednesday subpoenaed former national security adviser Mike Flynn for documents in its investigation into Russian interference in the 2016 election.

A statement from Senate Intelligence Committee Chairman Richard Burr, R-N.C., said the committee first requested the documents on April 28, but Flynn declined to cooperate. So now the committee is not asking.

Flynn was forced to resign as President Trump's national security adviser after revelations about how contact he had had with Russian officials was both far more extensive and more troublesome than had initially been thought by the White House.

Vice President Mike Pence defended Flynn only to learn that what he had said was not accurate.

Flynn's subpoena over the Senate-Russia probe comes one day after Trump fired FBI Director James Comey amid the FBI's own investigation into possible collusion between members of the Trump campaign and Russia.

The subpoena also comes after Sen. Richard Burr, chairman of the Senate Intelligence Committee said that he was "troubled" by the president's firing of Comey, calling the former FBI director a "public servant of the highest order" in a statement on Tuesday.

It is only the latest public setback for Flynn, who was forced to retroactively register as the agent of a foreign government for advocacy he did last year on behalf of Turkey.

It is unclear the extent to which he will cooperate with the Senate probe as he has in the past reportedly requested immunity from prosecution in exchange for congressional testimony.