The leading Democrat on a congressional committee has said documents suggest Michael Flynn lied to federal security clearance investigators about the source of payments he received from a Russian state-sponsored TV network.
Representative Elijah Cummings said Donald Trump's former National Security Adviser told the investigators during an early 2016 security clearance review that a trip to Moscow was "funded by US companies".
Mr Cummings said the actual source of the funds was "the Russian media propaganda arm RT".
He made the statements in a letter to Jason Chaffetz, the Republican chairman of the House oversight committee.
Mr Cummings's letter came the same day Mr Flynn declined to provide documents to the Senate Intelligence Committee, citing his Fifth Amendment protection from self-incrimination.
The top senior members of the Senate intelligence committee said they would "vigorously pursue" the evidence of Mr Flynn despite him invoking the Fifth Amendment.
Richard Burr and Mark Warner said they were disappointed that Mr Flynn had ignored the committee's subpoena.
Earlier this month, the committee asked Mr Flynn and other Trump associates for lists of meetings and notes taken during the presidential campaign.
The Senate intelligence committee is among the congressional panels investigating Russia's election meddling and possible ties with the Trump campaign. The FBI is also investigating.