U.S. securities regulators are probing whether market professionals violated securities laws during the recent unexplained market crash, Securities and Exchange Commission Chairman Mary Schapiro said on Thursday.

Schapiro said she was deeply concerned about the effects the volatile markets had on investors, in particular retail investors.

Two weeks after the Dow Jones industrial average dropped about 700 points within minutes, regulators are still trying to pinpoint the cause.

Schapiro reiterated the events of May 6 were unacceptable and her agency was taking necessary steps to implement safeguards.

(Reporting by Rachelle Younglai; Editing by Neil Stempleman)