Washington (CNN) House Democrats sent special counsel Robert Mueller what they say is evidence that former national security adviser Michael Flynn failed to disclose a trip he took to the Middle East to explore a business deal with the Saudi government and a Russian government agency.

The Democrats allege the retired Army lieutenant general broke the law by omitting the trip, according to the letter they sent to Flynn's former business partners requesting more information about his overseas travels and contacts.

The letter was sent by Maryland Rep. Elijah Cummings, the top Democrat on the oversight committee, and New York Rep. Eliot Engel, the top Democrat on the House foreign affairs committee. No Republicans from the two GOP-led committees signed onto the letter, a copy of which was also sent to Mueller.

When Flynn returned to the US several months later and reapplied for his security clearance, he never disclosed the trip or the contacts he had with foreign nationals as part of the trip, the Democrats say.

It is illegal under federal law to knowingly falsify or conceal relevant information from a security clearance form. The Democrats reviewed Flynn's paperwork and subsequent interview with the FBI -- routine practice to get a security clearance -- and said that Flynn never disclosed the summer 2015 trip.

"It appears that General Flynn violated federal law by omitting this trip and these foreign contacts from his security clearance renewal application in 2016 and concealing them from security clearance investigators who interviewed him as part of the background check process," according to the letter from the House Democrats to Flynn's business partners.

Robert Kelner, an attorney for Flynn, declined to comment for this story. But the former national security adviser's attorneys told the House Democrats that they would not respond to their inquiries, saying it would only do so under a "compulsory process," which would require support from House Republicans, according to the Democrats' letter.

Flynn traveled to Egypt and Israel in June 2015 to lobby officials there about the benefits of the proposal, according to a statement given to the Democrats by Dr. Thomas Cochran, a physicist who advised ACU Strategic Partners, one of the American companies involved in the deal.

"I joined General Flynn in Israel," Cochran said in his statement. "The primary purpose of the Israeli leg of the trip was to ensure that the ACU project architecture would be in the best interests of Israel. Because General Flynn firmly believed in the necessity of the project from a U.S. National Security perspective, he traveled to Egypt and Israel to explain the ACU project's importance."

An attorney for ACU Strategic Partners, one of the companies Cummings and Engel have reached out to, provided CNN with a copy of its letter explaining the virtues of the energy project, saying it would bolster national security and provide stability in the Mideast while improving "relations with Russia." A representative for the other company listed, X-Co Dynamics Inc./IronBridge Group, did not respond to a request for comment.

Flynn never disclosed these trips, and any conversations he might have had with government officials, on his security clearance forms when he reapplied in January 2016, the Democrats said.

The short-lived national security adviser also didn't initially disclose that he was paid for the trips, the Democrats say.

ACU's managing director Alex Copson said in a statement to the Democrats that they sent Flynn a check worth $25,000 "to compensate him for the loss of income and business opportunities resulting from this trip." Copson said Flynn never cashed the check, but Flynn accepted payment for travel expenses from ACU.

White House employees are required to publicly disclose details of their personal finances when they take government roles. Flynn submitted his first financial disclosure on February 11, and updated it on March 31, several weeks after he was fired. When he updated the form once more on August 3, Flynn indicated that he was paid more than $5,000 by ACU, according to the Democrats' letter.

Under the proposed deal, American consulting companies would partner with the Saudi government and Rosatom, Russia's government-run nuclear energy agency, to build 16 nuclear energy plants in Saudi Arabia. The Saudis would then sell that energy to eight other Sunni Arab countries, including Egypt and Jordan. As part of the deal, these countries would also buy military hardware from Russia, according to the companies' letters provided to Democrats.

Those arms sales would have likely been facilitated by Rosoboron, a Russian state-run weapons exporter, according to an internal presentation, published by Newsweek , from one of the companies involved in the deal. The Treasury Department sanctioned the Russian state-run company in September 2015 for violating US laws prohibiting weapons sales to Iran, Syria and North Korea.

The Democrats say the responses from the companies suggest they may have pursued the deal while Flynn was still national security adviser and with the Trump administration after he left the White House.

"Your responses suggest that you and other officials at your companies continue to strongly believe in the desirability of this project and that you may have discussed it with Trump administration officials during and after General Flynn's tenure at the White House," the Democrats said. Asked whether there were talks with the White House, an attorney for ACU referred CNN to a letter the firm sent to the Democrats earlier this summer, which does not address the topic.

In his initial February filing, Flynn also omitted thousands of dollars in payments he took from three Russian companies. One of those payments came from RT, the Kremlin-controlled TV network that US intelligence says pushed propaganda and was involved in Russia's interference in the 2016 election.

Cummings and Engel first raised the issue in June when they requested documents from Flynn's business partners. In that letter, they noted that Flynn did disclose a trip to Saudi Arabia that he took in October 2015, but they said he withheld key details and misled the FBI. For instance, Flynn said he went to speak at a conference, but the speakers' bureaus he worked for had no records of the trip.

A spokesperson for House Oversight Committee Chairman Trey Gowdy, R-South Carolina, declined to comment on the Democrats' letter. But a Gowdy aide said that the chairman wants his panel to steer clear of questions about violations of security clearances, saying that's a question for Mueller to explore. A committee spokesman for House Foreign Affairs Committee Chairman Ed Royce, R-California, said the Republicans had not received the letter as of Tuesday evening.

Flynn's company received subpoenas from the Senate and House intelligence committees earlier this year, as part of their investigation into Russian meddling. Flynn did provide documents to the House and Senate panels, but declined to testify, citing his Fifth Amendment rights

Flynn is under scrutiny by Mueller on several fronts, including his links to Russia, his calls to the Russian ambassador during the transition period, and the undisclosed lobbying he did for Turkey last year. Mueller issued grand jury subpoenas in the spring as part of his investigation into Flynn's dealings.