House Democrats are accusing former national security adviser Mike Flynn of illegally concealing several contacts with foreign officials and overseas trips while he was renewing his security clearances.

Reps. Elijah Cummings, D-Md., and Eliot Engel, D-N.Y., said members of Flynn's own team have provided information that confirmed Flynn visited the Middle East to meet with officials to promote a business deal to build nuclear reactors there.

"Based on your responses, 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," the Democrats wrote in a letter they released Wednesday morning. Cummings is the top Democrat on the House Oversight and Government Reform Committee, and Engel is the top Democrat on the House Foreign Affairs Committee.

Their letter said that because those violations can lead to criminal penalties of up to five years in prison, "we are providing your responses to Special Counsel Robert Mueller."

House investigators reached out in June to three American companies advised by Flynn after Newsweek reported that he traveled to the Middle East in 2015 to pursue a deal between the companies and Russia to bring nuclear power to several Middle Eastern countries. When Flynn reapplied for his security clearance several months later, he did not disclose the trip or his contacts with foreign nationals, the Democrats allege.

They also asked for all documents Flynn's team might have related to his contacts with any foreign government official related to the project.

Cummings said Flynn has shown he has hidden these and other facts about his work with foreign officials.

"He has, over and over again, omitted information that he should have disclosed," Cummings told ABC News. "It's not an aberration, and that's clear."

Flynn stepped down as White House national security adviser in February after he lied about his communications with the Russian ambassador to the U.S.