Ousted National Security Adviser Michael Flynn appears to have “lied” about a payment from Russian operatives when he was interviewed by investigators in the course of renewing his top-secret national security clearance, the House Oversight Committee’s ranking Democrat alleged Monday.

“The Oversight Committee has in our possession documents that appear to indicate that General Flynn lied to the investigators who interviewed him in 2016 as part of his security clearance renewal,” Rep. Elijah Cummings of Maryland in a letter to Chairman Jason Chaffetz (R-UT).

At issue is the $45,000 the retired lieutenant general was paid to speak at a 2015 Moscow gala put on by Kremlin-run news outlet RT, where he was seated next to Russian President Vladimir Putin. Cummings’ letter asserted that documents “the Oversight Committee has in our possession” show that Flynn told investigators that he was paid by “U.S. companies” when he traveled to Moscow for that gala, when the “actual source of funds” for the trip was RT itself.

Flynn also left a line blank on his security clearance renewal application that asked him to disclose any business relationships or transactions with foreign entities, a U.S. official with direct knowledge told NBC News, calling it a “lie of omission.”

Flynn’s ties to Russia are now at the heart of a federal investigation that focuses in part on possible collusion between Russian operatives and the Trump campaign to influence the U.S. election. Flynn refused Monday to comply with a subpoena request from the Senate Intelligence Committee for documents related to its own investigation into Russia’s interference in the 2016 election.

Knowingly accepting payments from foreign governments as a former military officer is a violation of the Emoluments Clause of the Constitution, and intentionally lying to federal investigators is a felony punishable by a sentence of up to five years imprisonment. Flynn faces further legal trouble for failing to report $530,000 he received to lobby for a Turkish businessman on his financial disclosure form—work that forced him to retroactively register as a foreign agent after leaving the White House.

The Trump administration has blamed former President Barack Obama’s team for failing to properly vet Flynn, as Cummings noted in his letter.

He urged Chaffetz to subpoena the White House for failing to honor their bipartisan request, first made in late March, for all documents relating to Flynn’s security clearance applications.

“We need to know what the President, Vice President, White House Counsel, and other top officials knew about General Flynn—and when they knew it,” Cummings wrote.

Read Cummings’ full letter below: