In addition to appearing on newscasts as a commentator, Flynn, the former head of the Defense Intelligence Agency, had attended a gala in Moscow marking RT's 10th anniversary in December 2015, where he sat next to Russian President Vladimir Putin.
Depending on the type of work that Flynn did for the two Russian firms - which the letter does not make clear - it may not be subject to disclosure to the Justice Department under the Foreign Agents Registration Act (FARA).
Cummings accused Trump and his advisers of showing unprecedented disinterest "in the truth" of Flynn's work on behalf of foreign clients - "whether due to willful ignorance or knowing indifference".
Kaspersky is one of the most prominent names in cybersecurity, but the firm has faced scrutiny in the past over founder Eugene Kaspersky's speculated ties to Russian intelligence agencies.
The US intelligence community has long assessed RT to be a propaganda tool of the Kremlin, writing in its January report on Russian interference in the US election that the organization had participated in disinformation campaigns aimed at the US.
Prosecutors Search Audi Offices in Germany
More than two million Audis had the manipulated software, including more than half a million in Germany alone. The engine is said to be used in several auto models of VW, Audi as well as Porsche.
Michael Flynn, who was forced to resign as national security adviser amid controversy over his contacts with Russia's ambassador, collected almost $68,000 in fees and expenses from Russia-related entities in 2015, a higher amount than was previously known, according to newly released documents.
Flynn served as Trump's national security advisor for less than a month before questions about his foreign ties sparked his dismissal. However, he said the source of the money was his U.S speakers' bureau and not the Russian broadcaster.
In December 2015, Flynn was flown out to Russian Federation to speak at a gala hosted by news outlet RT, which has been accused of being media arm of the Kremlin.
The White House has said Trump was unaware of Flynn's Turkey involvement, and Vice President Mike Pence has said he too did not know. "Come on!" said Flynn. As Business Insider reports, the event's other keynote speaker, U.S. Rep. Michael McCaul did not receive a payment to speak at the event. The company maintains that it is not affiliated with any government but is "proud to collaborate with the authorities of many countries, as well as worldwide law enforcement agencies in the fight against cybercrime". The biggest paycheck came from RT, which handed over $45,000, and during that evening, he also dined next to Vladimir Putin. The documents show that on October 14, an RT producer wrote she was "helping to organize the RT conference in December". She went on to write that: "We would be delighted if you would join our celebration and share your insights as part of the following session: "Geopolitics 2015 and Russia's changing role in the world". She added that the event would provide "exceptional networking opportunities for high-level media and political influencers from Russian Federation and around the world".