Flynn was forced to step down from his position in February after only 24 days amid revelations that he misled Vice President Mike Pence about his conversations with Russia's ambassador to the US, Sergey Kislyak.
The letter, released today by Cummings, was a primer on ethics restrictions that apply to retired military officers and it warned that Flynn, a retired lieutenant general, was prohibited from receiving foreign payments without prior approval, under the emoluments clause of the U.S. Constitution.
The White House defended its hiring of Flynn and attempted to shift blame for any problems with his vetting onto the Obama administration, which handled the reissuance of his security clearance in January 2016.
According to Cummings, the new documents also contradict Flynn's lawyer's claim on Tuesday that Flynn actually had received proper permissions from the Defense Intelligence Agency (DIA) before a trip to Moscow in December 2015.
Spicer also said the Trump administration welcomes the Department of Defense investigation of Flynn. The Oct. 8, 2014, letter - which was sent to Flynn at his request - explained that such "advance approval" would need to come "from the relevant service secretary".
Rep. Elijah Cummings (D-Md.), the ranking member of the House Oversight Committee, said Thursday that Flynn did not disclose the RT payment when he applied for a renewed security clearance in January 2016, a month after traveling to Russian Federation.
The oddest thing about this is that it's not news that Flynn received money for the RT appearance. He was paid at least $33,750 to attend the gala at which he was seated next to Russian President Vladimir Putin.
Flynn's attorney, Robert Kelner, said in a statement that Cummings mischaracterized the DIA letter, saying a partially redacted section confirmed that Flynn gave the Pentagon "information and documents on a thumb drive" about the RT event. Chaffetz said he believes the administration simply does not possess the documents, while Cummings has said they are intentionally withholding information about Flynn.
"DIA did not locate any records of LTG Flynn seeking permission or approval for the receipt of money from a foreign source", the agency wrote April 7.
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The issue involving Turkey emerged after Flynn retroactively registered in March with the Justice Department as a foreign agent for work that his company, Flynn Intel Group, carried out on behalf of Inovo BV. "We found no evidence that he did that".
The committee's leaders reported earlier this week that they found no evidence that Mr Flynn asked for permission for the payments he received or informed the military that he had accepted them. (His request was denied.) "He is now the target of unsubstantiated public demands by Members of Congress and other political critics that he be criminally investigated".
Flynn is just one component of at least four separate congressional investigations related to Russia's meddling in the USA election a year ago.
Before becoming his national security adviser, Flynn advised Trump's presidential campaign.
The probe, launched earlier this month, was revealed in a letter released Thursday by Rep. Elijah Cummings, the top Democrat on the House Oversight Committee.
"There's obviously a paper trail", he said.
"I honestly do not understand why the White House is covering up for Michael Flynn". He was sacked after misleading Vice President Mike Pence about discussions he had with Russian officials.
Spicer replied that Flynn's career military status required that he maintain a high level security clearance.
"His clearance was last reissued by the Obama administration in 2016 with full knowledge of his activities in 2015", Spicer said.