Regulators are still trying to ferret out what caused the Dow Jones industrial average to mysteriously drop nearly 700 points in minutes before sharply recovering, the chairman of the Securities and Exchange Commission said on Friday.

More than two months after the market briefly crashed in May, market regulators are still exploring a number of theories, including an imbalance between buyers and sellers.

To reduce the chances of another so-called flash crash, the SEC and exchange operators have implemented rules that would pause trading in a stock if markets are in freefall.

"We remain concerned about our markets' fairness, as well as their functioning," SEC chief Mary Schapiro told a conference in Chicago.

The SEC is also pushing tighter rules for market makers to ensure that there is liquidity during stressful times, sources said.

Even before the flash crash, the SEC was examining market structure issues with an eye toward ensuring a level playing field for both sophisticated and retail investors.

The agency proposed to ban flash orders, where exchanges flash buy and sell orders to some market participants before making them public. It has also proposed rules that would bar brokers from providing their unlicensed clients with unfettered access to public markets.

"We are working to ensure that accelerating technology and evolving trading strategies are not creating a two-tiered marketplace that leaves many investors at a significant disadvantage," Schapiro said.

At the same event, Schapiro said it was not the SEC's job to define what is good and bad governance for companies in a "one-size fits all approach."

"We believe investors' interests are served when they can participate productively in the governance of the companies they own," she said.

Her comments come as Democrats try to pass sweeping financial regulations designed, in part, to give shareholders more rights.

The agency will meet on Wednesday to discuss ways to improve the mechanics of corporate elections. The SEC is also under pressure from Democrats and shareholder activists to adopt rules that will give investors "proxy access," an easier and cheaper way to nominate corporate board directors.

The pending legislation affirms the SEC's authority to adopt proxy access rules. It is not clear whether the agency will do so before the bill is signed into law.