Sen. Elizabeth Warren, D-Mass., prodded Robert Cook, CEO of the Financial Industry Regulatory Authority, Thursday in a letter to “conduct a thorough review” of Robinhood’s activities during the GameStop trading frenzy.

In her letter, Warren also asked Cook to explain how FINRA will respond to Robinhood’s role “in recent market volatility, its decision to cut off customers’ trading, and the broader concerns about market fairness that these events represent.”