WASHINGTON (Reuters) - U.S. Treasury Secretary Janet Yellen on Thursday vowed to protect investors, but said financial market regulators needed to fully understand the recent trading frenzy involving GameStop Corp and other retail stocks before taking any action. The core infrastructure of financial markets had proven resilient during the high volatility and heavy trading volume around GameStop and other stocks, Yellen said after a high-level meeting with other top regulators to discuss recent market volatility. But a timely study of the events by the Securities and Exchange Commission was important, Treasury said, adding the SEC and Commodities Futures Trading Commission were reviewing whether the trading practices were consistent with investor protection and fair and efficient markets. Yellen told ABC’s “Good Morning America” earlier Thursday it was critical to ensure “that our financial markets are functioning properly, efficiently and that investors are protected.” In her first media interview, Yellen said President Joe Biden’s $1.9 trillion federal stimulus plan was needed to alleviate the economic pain caused by the coronavirus pandemic, which has left millions of Americans jobless. “We never had anything so large even during the Great Recession. We need to make sure people have jobs, if they don’t have jobs, that they’re supported,” Yellen said, referring to the 2007-2009 economic contraction in the United States. She said Biden still wanted Congress to pass the plan on a bipartisan basis and “is looking to cooperate” with Republicans. UNDERSTANDING WHAT HAPPENED Yellen convened the heads of the SEC, CFTC, the Federal Reserve Board and the Federal Reserve Bank of New York to discuss retail trading and “whether or not the recent events warrant further action,” she told ABC. “We need to understand deeply what happened before we go to action, but certainly we’re looking carefully at these events.” She did not specify what potential actions could be taken by regulators to respond to the situation. Many on Wall Street have been stunned this past week by the sharp gyrations in shares of video-game retailer GameStop, headphone maker Koss Corp, cinema chain AMC Entertainment and other stocks and commodities favored on the Reddit social media site’s Wall Street Bets forum. Traders had bid the shares to dizzying heights in an effort to punish shortsellers - who profit when shares fall - forcing some hedge funds to close their positions at heavy losses. But the so-called “Reddit Rally” later collapsed, exposing many individual traders to huge losses themselves. GameStop shares closed Thursday down 42% at $53.50, far from their peak of $483 a week ago. AMC Entertainment has lost about two-thirds of its value after two weeks of wild swings. Regulators likely discussed the online forums where mass buying of the stocks of those two companies was discussed last week, and the ever-larger role played by hedge funds in financial markets. “Any kind of market distortion by investors agreeing to cause the distortion goes against the smooth and transparent functioning of markets,” said Andrea Cicione, head of strategy at TS Lombard, adding that such activity has not been previously scrutinized by regulators. The SEC is reviewing social media posts for signs of potential fraud, Bloomberg News reported. Before calling the meeting, Yellen sought and received permission from Treasury ethics lawyers to do so and ensure that she was in compliance with her ethics agreement. Reuters reported here on Monday that such an ethics waiver might be necessary because of the over $700,000 in speaking fees Yellen was paid by Citadel LLC, a hedge fund that has been at the center of the GameStop trading saga. Citadel, whose trading arm profits from processing trades on the Robinhood commission-free trading app favored by many retail investors, had provided a $2.75 billion lifeline to Melvin Capital, a hedge fund that had suffered major losses in the GameStop short-squeeze.