By Tom LoBianco CNN

(CNN) -- Former national security adviser Michael Flynn has denied a request for documents from House Russia investigators and the House intelligence committee is preparing to issue subpoenas against Flynn, the ranking Democrat on that committee said Wednesday.

Rep. Adam Schiff, a California Democrat, also said investigators are seeking any audio recordings maintained by President Donald Trump of his private meetings in the White House and any memos maintained by Director of National Intelligence Dan Coats and National Security Agency Director Mike Rogers about Trump's reported request that they deny evidence of Russian interference in the US election.

"If all of this is part of a pattern of interference by the White House or worse, we have to find out," Schiff said at The Christian Science Monitor Breakfast in Washington.

"I said we would use whatever compulsory process we need ... if the subpoenas are not adhered to in a way that is supported by their legal right then I think we need to explore whatever compulsion Congress for obtaining" those documents, Schiff said.

The leaders of the Senate Russia investigation, Sens. Richard Burr and Mark Warner, said Tuesday that Flynn had refused to respond to their subpoena for documents and that, if he continues to refuse, they may hold him in contempt of Congress. Earlier this week, Flynn's lawyer said the former national security adviser would invoke his Fifth Amendment rights against self-incrimination in not complying with subpoenas from Senate investigators.

Schiff also cautioned that it was probably too early to begin talking about impeachment -- noting the legal and practical hurdles.

"Theres a lot of information that we need to gather about what the President is alleged to have done and what the President's intentions were, before we can reach any conclusions on the legal issues," Schiff said.

He added that as long as Republicans, who control the House, and the public at large believes impeachment is about sour grapes for Democrats, then it will be useless to press for it.

"For the evidence to get to the point where you even talk about impeachment, the Republican members would have to think the President's conduct is so serious where it disqualifies him for further service," Schiff said.

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