The head of the Securities and Exchange Commission says the agency is reviewing what caused a software glitch that threw the stock market into turmoil Wednesday, calling it "unacceptable."

SEC Chairman Mary Schapiro says technical problems like the one caused by Knight Capital Group illustrate how investor confidence can be shaken. The technical problem briefly put trading of dozens of stocks into chaos.

While the markets must rely on computers, regulators and market officials must still try to reduce the chances of technical errors and limit their impact when they occur, she said.

Schapiro said some of the trading controls put in as a result of the May 6, 2010 "flash crash" helped to limit the impact of Knight's error Wednesday.