The House oversight committee's top Democrat said Monday that documents he's reviewed suggest that former National Security Adviser Michael Flynn lied to federal security clearance investigators about the source of payments Flynn received from a Russian state-sponsored television network.

In a letter to the committee's chairman, Rep. Elijah Cummings, D-Md., says Flynn told investigators during an early 2016 security clearance review that a December 2015 trip to Moscow was "funded by U.S. companies." Cummings says the actual source of the funds was "the Russian media propaganda arm, RT."

Cummings went on to ask the chairman, Rep. Jason Chaffetz, R-Utah, to subpoena the White House for documents about Flynn that Cummings says have been withheld from the committee.

Cummings' letter was released the same day Flynn declined to provide documents to the Senate intelligence committee, citing his Fifth Amendment right against self-incrimination.

Late Monday, the chairman and ranking member of the Senate intelligence committee vowed they would "vigorously pursue" Flynn's testimony.

While we recognize General Flynn’s constitutional right to invoke the Fifth Amendment," Sens. Richard Burr, R-N.C., and Mark Warner, D-Va., said in a joint statement, "we are disappointed he has chosen to disregard the Committee’s subpoena request for documents relevant and necessary to our investigation.

"We will vigorously pursue General Flynn’s testimony and his production of any and all pertinent materials pursuant to the Committee’s authorities."

The Associated Press contributed to this report.