"At this event, the whistleblower met Alex Copson", Cummings wrote. But he asked Gowdy to issue a subpoena to the White House for documents related to Flynn, saying the committee has "credible allegations" that Flynn "sought to manipulate the course of global nuclear policy for the financial gain of his former business partners".
The letter said that if this witness account is accurate, Mr Trump's national security adviser "was trying to manipulate the course of global nuclear policy for the financial gain of his former business partners".
Cummings did not identify the witness, whom he described as a whistleblower.
"I believe the American people want Congress to hold President Trump and his administration accountable", Cummings wrote, "and they are exhausted of Republicans in Congress putting their heads in the sand when faced with credible allegations of grave abuses".
Trump Jr. spoke to the committee behind closed doors Wednesday.
The company did not respond to a request for comment Wednesday.
Flynn pled guilty last week to lying to the Federal Bureau of Investigation regarding conversations with Russians.
President Obama first imposed economic sanctions on Russian Federation in 2014, after Russia's military incursions in Crimea and Ukraine, and again last December to punish Russian Federation for its attempts to disrupt the United States presidential election.
Cummings wrote that Mueller's office had asked him to hold back the information until "they completed certain investigative steps" - which apparently have now been concluded.
Cummings said the whistleblower, whom he did not identify in the letter, "fears retaliation", but would speak with Gowdy if he agrees to protect the whistleblower's identity.
Flynn had assured his former associate that USA sanctions against Russian Federation would immediately be "ripped up" by the Trump administration, a move that would help facilitate the deal, the associate told the witness.
He greeted Mr Copson at the event, and asked how he was doing.
"I do not bring this whistle-blower to your attention lightly", Mr. Cummings said.
He added, "This is going to make a lot of very wealthy people".
The whistleblower said that Copson quickly displayed what he claimed to be a text message from Flynn that appeared to have been sent during Trump's inauguration speech.
Copson showed the witness the text on his phone.
The whistle-blower recalled the time stamp because it was during Mr Trump's address.
According to the whistleblower, Copson said, "Mike has been putting everything in place for us".
Mr Copson explained that Flynn was making sure that sanctions were going to be "ripped up" as one of the first orders of business and this would allow money to start flowing to the project. Flynn served as an advisor to the group between April 2015 and June 2016, according to his government disclosures, while it was advocating the nuclear reactor plan.
The letter from Cummings concluded with a request to issue subpoenas for "all documents and communications referring or relating" to Flynn's foreign contacts during the Trump campaign, the transition period and his brief time as national security adviser.
Mr Copson couldn't be located for comment. But it's not. Cummings provided hard evidence that he wants to form the basis of a subpoena created to uproot what really happened.
"I believe our committee can and should pursue these allegations against General Flynn in a responsible way in consultation with the special counsel's office", he wrote.