WASHINGTON - 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.

SEC Chairman Mary Schapiro says in prepared testimony for a House hearing Tuesday that the plunge "undermined confidence in the integrity of the financial markets."

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.

Markets design their own tools for slowing or halting trading, and regulate themselves.