The Senate's intelligence committee had asked Flynn - President Trump's former national security adviser - in April to turn over documents relevant to the committee's investigation into Trump campaign associates' connections to Russian Federation.
"General Flynn's attorneys have not yet indicated their intentions regarding the Senate Intelligence Committee's subpoena", the Republican chairman of the panel said in a statement.
When Flynn's lawyer, Robert Kelner, said Flynn would not be turning over the documents, the committee did what any good committee would do: It issued a congressional subpoena for the documents.
That same day, Flynn told friends, "I just got a message from the president to stay strong", Yahoo News reports. Richard Burr of North Carolina.
The Intelligence Committee has been probing Russia's alleged meddling the 2016 election, including whether members of the Trump campaign had colluded with Russian officials to benefit their candidate.
Flynn's refusal would put him in contempt of Congress, a crime for which the former national security adviser could be jailed up to 12 months.
The House intelligence committee is asking for more government documents - this time about the ouster of FBI Director Jim Comey and conversations he had with President Donald Trump about investigations into Russian meddling in the election.
Despite the disclosure, Trump appointed Flynn as his national security adviser.
Burr is leading a probe into Russian interference in the 2016 elections along with Sen.
The committee said it sent a letter requesting material from the FBI and Justice Department related to its ongoing counterintelligence investigation.
Congressional aides said negotiations with Flynn's lawyer continue, Reuters reported.
The district court route would entail the Senate passing a resolution authorizing civil action that could lead to Flynn being charged with contempt of court.