(CNN) Former national security adviser Michael Flynn was warned by the Defense Intelligence Agency in 2014 against accepting foreign payments as he entered retirement, according to new documents obtained by the House oversight committee.

The inspector general of the Department of Defense also opened an investigation of Flynn earlier this month, according to an April 11 letter released by Democrats on the oversight committee Thursday.

"These documents raise grave questions about why General Flynn concealed the payments he received from foreign sources after he was warned explicitly by the Pentagon," said Rep. Elijah Cummings, the top Democrat on the House oversight committee, in a statement. "Our next step is to get the documents we are seeking from the White House so we can complete our investigation. I thank the Department of Defense for providing us with unclassified versions of these documents."

The news comes two days after Cummings and House oversight chairman Jason Chaffetz said Flynn may have broken the law by not disclosing payments he received from RT-TV, a station widely considered to be a propaganda arm of the Russian government. Chaffetz also sent a letter Thursday to the acting secretary of the Army asking for a final determination as to whether Flynn violated the law.

Thursday's announcement also spotlighted an apparent division between the committee's two parties in the committee, which had so far appeared to be working closely together on the Flynn investigation. Chaffetz spokeswoman MJ Henshaw said Thursday that the release of the Flynn documents constituted a violation of trust by Cummings because he never consulted with Chaffetz before releasing them.

"Though we've walked hand-in-hand with the Democrats during this investigation, this morning they broke with long-standing protocol and decided to release these documents without consulting us," Henshaw told CNN.

Democrats, including a Cummings spokeswoman, pushed back on the idea they hadn't worked with Republicans on the committee.

"We consulted with them extensively over the past several weeks as part of the process of working with the Pentagon to prepare unclassified versions of the documents for public release, and they were included in multiple telephone and email communications with the Pentagon," Cummings spokeswoman Jennifer Werner said. "We informed them this morning that we would be releasing these documents today. Our internal committee protocols call for consultation, not consent."

White House press secretary Sean Spicer told reporters on Thursday that he thinks it is "appropriate" for the Defense Department inspector general to look into Flynn "if they think there is wrongdoing."

Spicer, in defense of the way Flynn was vetted, stood behind the fact that his clearances were approved under the Obama administration, not acknowledging that Trump was the one who decided to bring him into the administration.

Shortly after the briefing, President Donald Trump himself was asked by a reporter during a memorandum signing if he had any regrets about Michael Flynn.

Sitting at the resolute desk in the Oval Office, Trump looked directly at the reporter and said, "Thank you."

In a news conference following the documents' release, Cummings again sharply criticized the White House for not releasing documents related to Flynn.

"I honestly don't understand why the White House is covering up for Michael Flynn," Cummings said. "I don't get it, after the President fired him for lying — they should be bending over backwards to help us. It does not make any sense and it makes the American people think they have something to hide. There is a paper trail that the White House does not want our committee to follow it."

White House Director of Legislative Affairs Marc Short outlined in a letter earlier this week to the committee how it would not complete the request from the panel, referring some requests to the Department of Defense, saying the office doesn't have custody of some of the other documents or simply stating "we are unable to accommodate" others.

The Democrats on the House Oversight Committee also sent a request to Chaffetz Thursday that they schedule a meeting with the White House to demand the release of the Flynn documents they have asked for.

"To your credit, you have sought and obtained some documents relevant to this investigation. Now it is time to obtain additonal documents directly from the White House," the Democratic lawmakers wrote.

Flynn's lawyer, Robert Kelner, had previously said that Flynn briefed the DIA on his speech to RT and the payments, but Cummings said Thursday that another document that was declassified this week shows no evidence to support that statement. CNN has reached out to Flynn's attorney Thursday and have yet to receive a response.

Flynn resigned from being Trump's national security adviser in February, after it was revealed he misled Vice President Mike Pence over conversations he had with with Russian Ambassador Sergey Kislyak regarding sanctions. Before becoming his national security adviser, Flynn advised Trump's presidential campaign.

Flynn is just one component of at least four separate congressional investigations related to Russia's meddling in the US election last year. Flynn offered to speak with congressional investigators last month on the condition of being granted immunity from prosecution, an offer no committee has yet apparently taken Flynn up on.