Donald Trump’s former national security adviser is under formal investigation by the Pentagon for his apparently undisclosed paid speaking engagements in Russia, it emerged on Thursday.

Retired army lieutenant general Mike Flynn, a former Defense Intelligence Agency (DIA) chief, has attracted the official scrutiny for potentially failing “to obtain required approval” for a Moscow speech to RT, a state-controlled news channel that US intelligence considers an arm of Kremlin propaganda.

Acting Pentagon inspector general Glenn Fine disclosed the inquiry in an 11 April letter to the House oversight committee, which is also investigating Flynn. The committee released Fine’s letter on Thursday.

Additionally, newly disclosed DIA documents show that Flynn was warned of “criminal sanctions” in 2014 should he fail to obtain express official approval for accepting money for services rendered to a foreign government.

Just months after Flynn’s 2014 firing from DIA, the intelligence agency’s lawyers informed the retired general of ethics guidelines for his post-government career.

Among them, a letter dated October 2014 shows, was a warning that Flynn would need “advance approval from the [Army] secretary and the secretary of state before accepting employment, consulting fees, gifts, travel expenses, honoraria or salary from a foreign government.”

Violations of a host of ethics restrictions, including unapproved foreign compensation, “carry criminal penalties”, the DIA general counsel’s office warned.

Yet the DIA was unable to find records of Flynn seeking any approval, let alone obtaining it, a 7 April letter from DIA to the oversight committee showed. The committee also released that letter on Thursday.

Elijah Cummings, the top Democrat on the committee, accused the White House of covering up for Flynn through lack of compliance with the committee’s request for documents pertaining to Flynn’s payments from RT and his then-undisclosed lobbying work for clients tied to the Turkish government.

“I honestly do not understand why the White House is covering up for Michael Flynn. The president fired him for lying about his communications with the Russian ambassador. They should be bending over backwards to cooperate,” Cummings said at a Thursday press conference.

Flynn’s attorney did not immediately respond to a request for comment.

White House press secretary Sean Spicer later shifted the blame to Barack Obama’s administration for granting Flynn security clearance. “Every individual who came into this White House either applied for a security clearance or had one,” he told reporters. “Everyone in the government goes through the same SF-86 process – every single person – and so why would you rerun a background check on someone who was the head of the department of the Defense Intelligence Agency that had and did maintain a high-level security clearance?

“There’s no difference between administrations – when you come in from one they rerun it. The reason they grant them for five years is that it’s a very extensive background, where they check your contacts, your places of residence, your employment. They go out into the field, they do a lot of that work, and then you’re required to maintain updates to that clearance. They re-adjudicate it every five years. That occurred in this case and now the Department of Defense inspector general’s looking into it.”

Spicer said he was “frankly taken aback” by Cummings’ allegations of a cover-up, insisting it was not true and that all relevant documents had been provided. Asked if Trump still believes that Flynn should receive immunity in exchange for his testimony, the press secretary said only: “I have not asked him, but I believe that General Flynn should do what his counsel advises him to do.”

Trump fired Flynn, his major military surrogate on the campaign trail, in February after Flynn ostensibly lied to the vice-president, Mike Pence, over conversations Flynn had with Russian ambassador Sergey Kislyak concerning easing sanctions on Moscow. The firing after 24 days in office made Flynn the shortest-serving national security adviser in history.

Shortly after his firing, Flynn retroactively registered as a foreign agent with the Department of Justice. He disclosed 2016 lobbying work that “could be construed to have principally benefited the Republic of Turkey” earning him over a half million dollars. Flynn was a Trump campaign surrogate during the August-to-November lifespan of the lobbying work.

In December 2015, Flynn attended and spoke before a gala celebration of RT’s 10th anniversary. He was seated to the right of the Russian president, Vladimir Putin, at the event. Flynn’s Moscow trip has become a subject of myriad investigations into ties between Russia and Trump’s campaign underway in Congress and the FBI.

Asked about the speech in August 2016, Flynn acknowledged it was “paid” but declined to specify the amount. The House oversight committee subsequently obtained and disclosed that he received over $67,000 from Russian firms, including $33,750 for the RT speech.

Through his lawyer, Flynn has offered to testify to the FBI and the House and Senate intelligence committee investigations, but only after obtaining a grant of immunity against subsequent prosecution. Thus far, the congressional committees have denied discussing any such immunity deal.