The enforcement division of the Securities and Exchange Commission is investigating whether market makers and brokerage firms fulfilled their legal obligations to provide liquidity in the markets by buying and selling stock during the sharp market drop of May 6, the chairwoman of the agency said Thursday. The SEC is also looking into whether market makers and brokers executed investors' trades correctly. Mary L. Schapiro, SEC chairwoman, said the agency was also considering whether to establish market participation mandates for professional traders like high-frequency traders who were not obligated to continue trading during periods of extreme market stress. The comments came during testimony to the Senate Subcommittee on Securities, Insurance and Investment, which, like a House subcommittee last week, had called the leaders of various market regulators and exchanges to testify on the causes of the May 6 market plunge. On May 6, stock prices fell by about 6 percent in a matter of minutes before recovering nearly as quickly. Earlier this week, the SEC said that in reaction to the market tumult, circuit breakers would be installed for individual stocks in the Standard & Poor's 500-stock index on a six-month test basis beginning in June. Those circuit breakers would halt trading for five minutes in stocks that fell or rose by more than 10 percent in a five-minute period. Schapiro said the commission would also re-examine its rules concerning circuit breakers for the overall market, which were not tripped on May 6 because the Dow Jones industrial average did not drop by 10 percent. In addition to reviewing whether that percentage level was appropriate, Schapiro said, the agency would examine whether the trigger should be based on a broader stock index, like the Standard & Poor's 500. Gary Gensler, the chairman of the Commodity Futures Trading Commission, said that his agency would also examine its circuit breakers, as well as whether there should be new rules governing the use of computer-driven, or algorithmic, trading. Gensler noted that while human traders could react to an unusual event like the one that occurred on May 6, computers simply did what they were instructed to do, repeatedly. That, he added, was part of the problem on May 6. During the period of highest market stress on May 6, many institutional investors stopped trading, according to a review of the day's trading activity by the SEC and the CFTC. Among those that legally stepped back from the market during that time were several of the firms that employ computer programs to trade millions of shares a second and that are usually the biggest providers of liquidity in the stock market. Schapiro said the loss of the most active traders came as the market experienced a flood of short sales, or bets that stocks would decline further, and 'stop loss' orders from institutional and individual investors, orders to sell shares at whatever market price was available. Together, those created a sort of black hole into which stock prices fell. The agency's divisions of inspections and enforcement have joined the SEC's review of May 6 trading. 'It appears that some investors' accounts were liquidated as shares plummeted only to have stock prices close significantly above their sale prices,' Schapiro said. 'We will examine such things as whether market professionals fully met their obligations, including, where applicable, their best execution obligations, and whether the decision to bust trades was made and applied fairly and consistently among investors,' Schapiro said. Thousands of trades were busted, or canceled, after May 6, for falling outside a certain price range during the period of intense volatility. That helped those investors who sold at what turned out to be unreasonably low prices, but it disadvantaged those who stepped into the market to buy at the height of the crisis, when few others would. Sen. Jim Bunning of Kentucky, the ranking Republican on the subcommittee, said he believed that the trade cancellations were unfair and served 'to undermine market discipline.' Schapiro said she had asked the stock exchanges to draft within the next two weeks a unified plan for determining when trades were erroneous and should be canceled. Not all stocks fell sharply on May 6. Some traded at unusually high prices. Five stocks traded for $100,000 a share, Schapiro said, a result of so-called stub prices that some market makers had entered into the system. The SEC is considering outlawing the use of stub quotes. 