Former National Security Advisor Michael Flynn appears to have failed to report a 2015 trip to Saudi Arabia on behalf of a U.S./Russia business plan to build nuclear reactors, according to a congressional letter issued Monday requesting documents from the companies he allegedly represented.

The seven-page letter — which contains the signatures of Democratic ranking members of the House Oversight and Foreign Affairs committees, but no Republicans — is framed as a request to three companies Flynn allegedly represented in the effort. The joint venture, which the letter notes was first noted in a Newsweek article, involved U.S. companies, a Russian state-sponsored company and Saudi financing, and was geared towards providing “nuclear power to the Arab world.”

In recent months, Flynn has been a common target of congressional inquiries looking into Russian interference in the 2016 election and the possible collusion by President Donald Trump’s campaign.

The letter questions why he failed to mention one trip, and underreported a second, on a January 2016 application for the renewal of his federal security clearance. It also questions why Flynn failed to mention “any of these contacts with Saudi or other foreign officials on his security clearance application or during his interview with security clearance investigators.”

The letter does not directly state that Flynn could face jail time for failing to provide this information, but does quote a federal criminal law regarding the federal security clearance applications, explaining that “knowingly falsifying or concealing a material fact is a felony which may result in fines and/or up to five years imprisonment.”

Flynn’s attorney, Robert Kelner, of Covington & Burling, declined to comment.The letter says Flynn testified about a Saudi Arabia trip to the House Foreign Relations Committee, but failed to report any foreign business interests in the matter.

The letter also raises the specter of Flynn’s alleged ties to Russia. It explains that not long after his summer 2015 trip to Saudi Arabia to talk about nuclear power plants, the Saudis made a $100 billion deal with the Russian state nuclear corporation, Rosatom, to build 16 nuclear power units. The letter does not indicate whether Flynn received any payment for the deal.

“It does not appear that General Flynn disclosed this trip or any foreign contacts as part of his security clearance renewal process,” the letter states.

The letter also addresses a second trip to Saudi Arabia, noting that Flynn did report a later trip, in October 2015. But the letter says Flynn “omitted key details.”

The letter states that he reported traveling to Saudi Arabia with a friend for six days to speak at a conference. He claimed he stayed at the King Khaled International Hotel and that his expenses were paid by a “work sponsor.”

“Unfortunately, Gen. Flynn did not disclose... what type of ‘business’ he had in Saudi Arabia, who his U.S. ‘friend’ was who accompanied him, what ‘conference’ he attended, or which ‘work sponsor’ paid for his expenses.”

The letter also indicates that the hotel does not appear to exist.

“Most troubling of all, we have no record of Gen. Flynn identifying on his security renewal clearance application — or during his interview with security clearance investigators — even a single foreign government official he had contact with in the seven years prior to submitting” the form.

The congressmen sent the letter to the Flynn Intel Group, X-Co Dyanmics Inc/IronBridge Group Inc, and ACU Strategic Partners, all of whom the letter says were involved with Flynn and the Saudi nuclear effort. According to a Bloomberg business profile, “ACU Strategy Partners LLC operates as a holding company for development of nuclear energy.”

These are not the first allegations that Flynn failed to report income and contacts before becoming the short-serving National Security Advisor. Back in April, this same committee said Flynn probably broke federal law by failing to disclose on a 2016 security clearance application that he had done business with Russia in 2015.

At that time, the committee chairman was Rep. Jason Chaffetz, R-Utah. He said documents the committee had requested indicated Flynn apparently had violated federal requirements that he seek permission from both the Pentagon and the State Department for a business arrangement with Russia. Chaffetz said neither department could find any evidence that Flynn had done so, adding that Flynn could face jail time.

Flynn has been at the center of controversy surrounding allegations of Trump officials colluding with Russia for months. Former Acting Attorney General Sally Yates testified before Congress in May that during the first days of Trump’s presidency she had two face-to-face and two phone conversations with the White House to discuss a “national security adviser compromised by the Russians.”

Yates testified that she spoke with White House counsel Donald McGahn about Flynn’s issues two weeks before he was removed from the position. That time, the concern was that Flynn was subject to blackmail by Russia because it had become clear he was lying about his Dec. 29 conversation with Russian Ambassador Sergey Kislyak. She said U.S. intelligence agencies, and probably the Russians, had recorded the conversation.

Beyond that, Flynn has been under scrutiny because even before he took office this year — and without disclosing that he had been paid $500,000 by a company to work on behalf of Turkey’s interests in 2016 — he told President Barack Obama’s national security adviser, Susan Rice, to hold off on a Pentagon plan to retake the Islamic State’s de facto capital of Raqqa. The plan included working with Syrian Kurdish forces, and Turkey was known to strongly object to training or arming Kurds in the region.

In this case, the letter discusses that part of what the congressional committees are seeking is what they don’t know they don’t know.

“The scope of General Flynn’s unreported foreign contacts and foreign financial interests is unclear from the time he left the Defense Intelligence Agency in 2014 until President Trump appointed him… in 2017,” it states. Flynn was the DIA director under President Barack Obama.