WASHINGTON DC, United States — The head of the Securities and Exchange Commission says last week’s massive market plunge was “unacceptable” and that the agency is committed to fully understanding what caused the turmoil.

The rapid drop “undermined confidence in the integrity of the financial markets,” SEC Chairman Mary Schapiro says in prepared testimony for a House hearing yesterday. Changes need to be considered to prevent severe disruptions in the future, she says.

Schapiro says a stronger and more uniform system for slowing trading during periods of high volatility would help. But she does not call for an expanded federal role in overseeing the exchanges.

Most of the 50 US exchanges regulate themselves and design their own tools for slowing or halting trading.

Executives of the New York Stock Exchange and Nasdaq plan to tell a House panel that they are capable of policing themselves. But they agree that broad changes to the system of overseeing the financial exchanges are needed.

Larry Leibowitz, chief operating officer for NYSE, says in his prepared testimony that a more uniform system of “circuit breakers” would slow trading when share prices plummet. He also says one self-regulator with access to all trading data might best oversee all the exchanges. The vast network of self-regulators has made it harder for SEC officials to investigate the chaotic trading, Leibowitz says.

“Any single exchange has access to only the data from trades sent to or executed on that exchange,” Leibowitz says in his testimony. “When a trading problem occurs . . . there is no central mechanism to coordinate a market-wide response.”

During Thursday’s plunge, NYSE slowed trading but other exchanges traded elsewhere, says Eric Noll, executive vice president of the NASDAQ OMX Group Inc.

Those events contributed to Thursday’s rapid sell-off that pushed the Dow Jones industrial average down nearly 1,000 points, Noll says in his prepared testimony. The market recovered to close down 342 points.

Another trigger of the sell-off was “aggressive, nervous selling” by investors who believed stock prices would continue to fall, Noll says. The Dow already was down 272 points at 2:35 pm Thursday, before the selling frenzy occurred.

Prices dropped rapidly for the “E-Mini June,” a popular security that tracks the expected value of the Standard & Poor’s 500 Index for June 2010, Noll says. He says that led to steep price drops for related stocks, such as Proctor and Gamble.

Those declines led NYSE to stop electronic trading for dozens of stocks. Human traders then handled the transactions — a much slower process.

As orders were routed to markets that lacked NYSE’s circuit breakers, “a spasm of selling spread through the markets with . . . no opportunity to pause or human judgment to intervene,” Leibowitz says.

Officials from NYSE, Nasdaq and other major exchanges agreed in a meeting with regulators Monday to propose specific circuit breakers and other measures to curb market turbulence.