The Justice Department is investigating high-frequency trading on Wall Street, Attorney General Eric Holder told a House committee today.

Holder appeared before the House Appropriations Committee today at a hearing on the budget. He told lawmakers that the Justice Department is investigating high-frequency trading to see whether it violates insider trading laws.

“This practice, which consists of financial brokers and trading firms using advanced computer algorithms and ultra-high-speed data networks to execute trades, has rightly received scrutiny from regulators,” he said. “The department is committed to ensuring the integrity of our financial markets – and we are determined to follow this investigation wherever the facts and the law may lead.”

High-frequency trading has become a hot issue in recent days since author Michael Lewis appeared on CBS’ 60 Minutes Sunday to discuss his new book, Flash Boys: A Wall Street Revolt, about computer-driven stock trading. Lewis says the U.S. stock market is rigged by a combination of insiders — stock exchanges, big Wall Street banks and high-frequency traders — who can move faster than other investors.

High-powered computers and advanced computer software allow high-frequency traders to — in milliseconds — step in front of other investors’ trades and make essentially riskless profit, he says.

“The market moves at two speeds: one speed for people who pay for access to the exchanges, who put their trading machines right next to the black boxes … and everybody else,” Lewis said Tuesday on the Today show. “And we are everybody else, everybody else being investors in the stock market.”

Though Lewis’ book brings new awareness to the topic, insiders have debated the pros and cons of high-frequency trading for months. On May 6, 2010, a market plunge of 1,000 points — and the bounce-back — brought the potential downside of increased market computerization to the attention of investors and regulators. Regulators have found it challenging to monitor and track the high-speed movements.

This week, the FBI confirmed it was probing high-frequency and high-speed trading on Wall Street. The Securities and Exchange Commission said it, too, was looking at high-frequency trading. New York Attorney General Eric Schneiderman has been investigating whether practices at the exchanges give traders unfair advantage over other investors.

The Modern Markets Initiative, a high-frequency trading group, denied Lewis’ charges. “The markets are not rigged. Saying otherwise is a broad generalization that lumps the vast amount of good market behavior in with a few bad actors,” the group said in a statement.