According to The New York Times, House Oversight Committee ranking Democrat Elijah Cummings wrote to committee chair Trey Gowdy that "within minutes of Donald Trump being sworn in as President", Flynn was "communicating directly with his former business colleagues about their plan to work with Russian Federation to build nuclear reactors in the Middle East".
The claims are detailed in a letter by Elijah Cummings, top Democrat on the House oversight committee.
The project in question - promoted by a group of former senior USA military officers, and often described as a "Marshall Plan" of sorts - would involve USA companies working with Russian companies to build and operate nuclear plants in the Middle East, and export spent fuel from those plants.
According to Cummings, on January 20, 2017, the whistleblower had a conversation with Copson as they attended Trump's inauguration. These grave allegations compel a full, credible, and bipartisan congressional investigation, ' Cummings wrote.
Neither Gowdy nor Flynn's attorney, Robert Kelner, responded immediately to a request for comment.
ACU Strategic Partners paid for Flynn to travel to Israel and Egypt in 2015 to promote the nuclear project, paying him a fee of $25,000, according to a letter Copson sent the committee.
"Mr. Copson explained that Gen. Flynn was making sure the sanctions would be ripped up as one of his first orders of business and this would allow money to start flowing into the project", Cummings said, quoting the witness.
That's the explosive, but unverified, allegation of a whistleblower cooperating with House Democrats probing the myriad scandals surrounding Flynn, who pleaded guilty earlier this month to lying to the Federal Bureau of Investigation about his contacts with a senior Russian diplomat.
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Flynn had texted Copson with the same message - that the nuclear reaction project was "good to go" - from his place on the dais outside the Capitol 10 minutes into Trump's inaugural address, the whistleblower told Cummings.
The White House aide was said to have been referring to plans by a consortium of US, Russian and French companies to build and operate 45 nuclear power plants in Saudi Arabia and other Arab countries.
Copson showed the witness the text on his phone.
The congressman said he had heard about the alleged Flynn texts from an unnamed confidential informant.
The office of special counsel Robert S. Mueller III was aware of the witness's account and asked Cummings not to release the information until the special counsel had taken "certain investigative steps", which are now complete, Cummings wrote.
The Wall Street Journal's previous reporting said Flynn had facilitated a draft memo with the National Security Council staff that would support the project.
Cummings did not name the whistleblower in the letter, but offered to have Gowdy speak with the person directly.
It also raised fresh questions on what Trump knew about Flynn's business plans when he appointed the retired three- star general to serve as his national security advisor.