Gen. Michael Flynn(C), former national security adviser to US President Donald Trump, leaves Federal Court in Washington, DC, December 1, 2017. Saul Loeb | AFP | Getty Images

President Donald Trump's former national security advisor Michael Flynn told a business associate that he would make sure economic sanctions against Russia would be "ripped up" as one of his first orders of business in the White House, a whistleblower told congressional investigators. Flynn told the business associate that a project they were working on to build nuclear reactors in the Middle East was "good to go" and directed him to push forward on the plan, the whistleblower said, according to a letter written by Rep. Elijah Cummings, the top Democrat on the House Oversight Committee. The whistleblower's account is included in a letter from Cummings to Republican Chairman Trey Gowdy. Cummings wrote that an "authentic, credible, and reliable" source had brought him information regarding the retired Army lieutenant general.

On Friday, Flynn pleaded guilty to lying to the FBI in a voluntary interview conducted only four days following his alleged text messages to Copson. He is now cooperating with special counsel Robert Mueller, who is investigating ties between Trump's top advisors and Russia. "Mike has been putting everything in place for us," Copson said, according to the whistleblower. He added, "This is going to make a lot of people very wealthy."

Committee investigation

Cummings' five-page letter to Gowdy calls for a slew of new subpoeanas and castigates the committee chairman for his "refusal to investigate." The House Democrat called for subpoenas to be issued to White House chief of staff John Kelly, Flynn, Copson and others. "When you took over as Chairman this summer, you refused to press the White House to comply with our bipartisan request for documents, and you disregarded new evidence we uncovered about General Flynn's secret effort to work with Russia on a plan to build nuclear reactors in Saudi Arabia," he wrote. Gowdy did not immediately respond to a request for comment from CNBC.

WATCH: Trump feels badly for General Flynn