Former Trump adviser Mike Flynn under DOD investigation, had been warned not to take foreign payments

— -- Retired Lt. Gen. Michael Flynn was warned by the Pentagon against receiving payments from foreign governments in 2014 after leaving the Defense Intelligence Agency, Rep. Elijah Cummings, D-Md., revealed today.

Cummings, the top Democrat on the House Oversight Committee, released three documents on Flynn, including a letter from the DIA counsel's office in response to an inquiry from Flynn in October 2014.

The letter, a primer on ethics restrictions that apply to retired military officers, warned that Flynn was prohibited from receiving foreign payments without prior approval, under the emoluments clause of the U.S. Constitution.

"The Pentagon's warning to Gen. Flynn was bold, italicized and could not have been clearer," Cummings said at a news conference today.

After the release of the letter, Rep, Jason Chaffetz, R-Utah, chairman of the House Oversight Committee, formally requested that the secretary of the Army make a final determination on whether Flynn broke the law and, if he has, then initiate the process to get Flynn to repay the money.

Flynn, who was President Trump's first national security adviser, was fired after it was discovered that he misled Vice President Mike Pence about conversations he had with a Russian official.

Cummings also revealed today that the Defense Department's inspector general has opened an investigation into whether Flynn sought permission to receive foreign payments, including payments in exchange for an appearance in Russia.

Another document, an unclassified letter from the Defense Intelligence Agency, indicates that the DIA "did not locate any records" relating to his receiving foreign payments or any records that he sought permission to do so.

"These documents raise grave questions about why Gen. Flynn concealed the payments he received from foreign sources after he was warned explicitly by the Pentagon," Cummings said in a statement.

Cummings called on the White House to provide documents on Flynn requested by the House Oversight Committee as part of its investigation into him.

"There's obviously a paper trail," Cummings said.

A Pentagon spokesman confirmed that on April 4 the Defense Department's inspector general began an investigation into Flynn's alleged violations of the emoluments clause.

Flynn's lawyer Robert Kelner said in a statement, "We respectfully disagree with Representative Cummings’ characterization of the April 7, 2017 letter from DIA to the Committee. DIA’s letter actually confirms, in a terse section that is partly redacted, that General Flynn provided information and documents on a thumb drive to the Department of Defense concerning the RT speaking event in Moscow, including documents reflecting that he was using a speakers’ bureau for the event.”

"General Flynn provided two briefings to the Department -- one before and one after the event. The Department was fully aware of the trip. We urge DIA and the Committee to release the full, unredacted letter, along with the documents that General Flynn provided to DIA during the briefings and details concerning the in-person briefings provided by General Flynn to DIA.”

ABC News' Luis Martinez contributed to this report.