WASHINGTON (Reuters) - The head of trading app Robinhood and Wall Street hedge fund managers on Thursday fielded a barrage of questions in the U.S. Congress about their role in the GameStop trading frenzy, at times being cut off by irate lawmakers who found their answers wanting. Financial heavyweights including billionaire Republican mega-donor and Citadel CEO Ken Griffin defended their businesses at the hearing, which lasted more than five hours. Lawmakers probed how Reddit users trading on retail platforms squeezed hedge funds that had bet against shares of the video game retailer and other companies. “Robinhood owns what happened and we need to make sure it doesn’t happen again,” said Robinhood CEO Vlad  Tenev, who appeared to field the most questions. Robinhood and other trading apps drew fire for putting trading limits on GameStop a day after the stock price more than doubled. The trading platforms, who face numerous lawsuits, said the limits were needed because the massive volatility prompted clearing houses that guarantee trades to call for billions of dollars in extra collateral. But outraged lawmakers questioned if the platforms were siding with hedge funds over retail investors. Representative Sean Casten, an Illinois Democrat, played an automated message on the Robinhood customer service phone line while questioning Tenev about how customers are treated. ADVERTISEMENT Griffin got numerous questions which focused on his company’s heft. Melvin Capital CEO Gabriel  Plotkin and Reddit CEO Steve Huffman also appeared. Keith Gill, a Reddit user and YouTube streamer known as Roaring Kitty, sat in a red videogaming chair from which he recorded many videos promoting his investment in GameStop. Displayed behind him was a poster of a kitten with the caption “Hang in there!” Many investors who bought GameStop as the price skyrocketed now face deep losses. “A few things I am not. I am not a cat. I am not an institutional investor, nor am I a hedge fund,” Gill said. He told lawmakers he was still enthusiastic about the stock. “I am just an individual whose investment in GameStop and posts on social media were based upon my own research and analysis,” he said. Representative Maxine Waters, chair of the Democratic-led House finance panel, is a critic of Wall Street, while Republicans on the panel also sought to burnish their consumer-protection credentials, making for a fiery virtual hearing featuring echoes, feedback and background noise. RELATED COVERAGE GameStop frenzy sparks fresh investment in stock-trading apps GameStop briefly rallies after "Roaring Kitty" testifies to Congress See more stories Waters, who in January called hedge funds “unethical,” frequently talked over Tenev and Griffin and cut them off. At the end, she vowed to hold more hearings, with testimony from regulators at the Securities and Exchange Commission and the Financial Industry Regulatory Authority. She said she was “more concerned than ever” that retail investors were being taken advantage of, and that large market makers like Citadel could pose a threat to the financial system. In prepared opening testimony, the witnesses generally acknowledged the GameStop saga was unprecedented, but they all said there was no foul play on their part. Graphic: GameStop"s rise and fall GameStop"s rise and fall - Reuters Graphic Graphic: GameStop comments soared on Reddit"s Wallstreetbets - Reuters Graphic Tenev expressed regret that Robinhood’s actions upset customers but said no hedge funds influenced its decision and the situation would have been “significantly worse” if it had not suspended buying. “I’m sorry for what happened. I apologize. I’m not going to say that Robinhood did everything perfect,” he said. New York-based hedge fund Melvin Capital, which had bet GameStop’s shares would tumble, suffered massive losses as the stock soared leading up to Jan. 28. Citadel made a $2 billion investment in Melvin on Jan. 25. FILE PHOTO: GameStop logo is seen in front of displayed Reddit logo in this illustration taken February 2, 2021. REUTERS/Dado Ruvic/Illustration/File Photo/File Photo Lawmakers quizzed Robinhood’s commercial arrangements with market makers to which it routes orders, as well as whether that “payment for order flow” model hurts customers. Griffin, majority owner of Citadel Securities, a Robinhood market maker, was castigated by Democratic Representative Brad Sherman for evading his question about whether market-makers provide the same prices to all brokers. Griffin maintained Citadel Securities, which executes roughly 47% of all U.S.-listed retail volume, had worked for years to help give consumers “a better price.” All parties have denied any attempt by Citadel or Citadel Securities to influence Robinhood’s policies. MANIPULATION Gill and Huffman were asked about the role of social media platforms in potential market manipulation. “We spend a lot of time at Reddit ensuring the authenticity of our platform... And in this specific case, we did not see any signs of manipulation,” Huffman told lawmakers. At times, Gill and Plotkin offered their competing investment theses on GameStop, with Plotkin arguing that due to the rise of digital downloads, the video game bricks-and-mortar business model has “structural challenges.” Reporting by Michelle Price, Pete Schroeder, Noel Randewich; John McCrank, Elizabeth Culliford, Svea Herbst-Bayliss, Megan Davies Editing by Megan Davies, David Gregorio, Steve Orlofsky and Dan Grebler Our Standards: The Thomson Reuters Trust Principles. MORE FROM REUTERS ADVERTISEMENT TRENDING STORIES Researchers urge delaying Pfizer vaccine"s second dose as first highly effective AstraZeneca vaccine faces resistance in Europe after health workers suffer side-effects After blizzard of criticism over Mexico trip, Senator Ted Cruz flies back to frozen Texas Printed in days, a house: New York firm takes 3D printing to the next level Facebook "unfriends" Australia: uproar as news pages go dark REUTERS NEWS NOW Subscribe to our daily curated newsletter to receive the latest exclusive Reuters coverage delivered to your inbox. Enter email address Submit AppsNewslettersAdvertise with UsAdvertising GuidelinesCookiesTerms of UsePrivacy All quotes delayed a minimum of 15 minutes. See here for a complete list of exchanges and delays. © 2021 Reuters. All Rights Reserved. Live: NASA holds a news briefing after Perseverance makes historic Mars land Griffin got numerous questions which focused on his company’s heft. Melvin Capital CEO Gabriel  Plotkin and Reddit CEO Steve Huffman also appeared. Keith Gill, a Reddit user and YouTube streamer known as Roaring Kitty, sat in a red videogaming chair from which he recorded many videos promoting his investment in GameStop. Displayed behind him was a poster of a kitten with the caption “Hang in there!” Many investors who bought GameStop as the price skyrocketed now face deep losses. “A few things I am not. I am not a cat. I am not an institutional investor, nor am I a hedge fund,” Gill said. He told lawmakers he was still enthusiastic about the stock. “I am just an individual whose investment in GameStop and posts on social media were based upon my own research and analysis,” he said. Representative Maxine Waters, chair of the Democratic-led House finance panel, is a critic of Wall Street, while Republicans on the panel also sought to burnish their consumer-protection credentials, making for a fiery virtual hearing featuring echoes, feedback and background noise. Waters, who in January called hedge funds “unethical,” frequently talked over Tenev and Griffin and cut them off. At the end, she vowed to hold more hearings, with testimony from regulators at the Securities and Exchange Commission and the Financial Industry Regulatory Authority. She said she was “more concerned than ever” that retail investors were being taken advantage of, and that large market makers like Citadel could pose a threat to the financial system. In prepared opening testimony, the witnesses generally acknowledged the GameStop saga was unprecedented, but they all said there was no foul play on their part. Graphic: GameStop"s rise and fall GameStop"s rise and fall - Tenev expressed regret that Robinhood’s actions upset customers but said no hedge funds influenced its decision and the situation would have been “significantly worse” if it had not suspended buying. “I’m sorry for what happened. I apologize. I’m not going to say that Robinhood did everything perfect,” he said. New York-based hedge fund Melvin Capital, which had bet GameStop’s shares would tumble, suffered massive losses as the stock soared leading up to Jan. 28. Citadel made a $2 billion investment in Melvin on Jan. 25. Lawmakers quizzed Robinhood’s commercial arrangements with market makers to which it routes orders, as well as whether that “payment for order flow” model hurts customers. Griffin, majority owner of Citadel Securities, a Robinhood market maker, was castigated by Democratic Representative Brad Sherman for evading his question about whether market-makers provide the same prices to all brokers. Griffin maintained Citadel Securities, which executes roughly 47% of all U.S.-listed retail volume, had worked for years to help give consumers “a better price.” All parties have denied any attempt by Citadel or Citadel Securities to influence Robinhood’s policies. Gill and Huffman were asked about the role of social media platforms in potential market manipulation. “We spend a lot of time at Reddit ensuring the authenticity of our platform... And in this specific case, we did not see any signs of manipulation,” Huffman told lawmakers. At times, Gill and Plotkin offered their competing investment theses on GameStop, with Plotkin arguing that due to the rise of digital downloads, the video game bricks-and-mortar business model has “structural challenges.”