(Bloomberg News) Knight Capital Group Inc. told some clients of its market-making unit that a "technical issue" was affecting its systems and advised them to route orders elsewhere as dozens of U.S. stocks swung more than 10 percent today.

Knight, which helps execute billions of dollars in equity transactions every day, said the issue was confined to market making and other operations were unaffected. Its stock plunged as much as 26 percent as investors speculated on its role in the incident, which spurred concern that computers had distorted trading for the second time in two weeks.

The incident, occurring after three Dow Jones Industrial Average stocks fluctuated in regular hourly patterns for a full trading day on July 19, may embolden critics of American market structure who say the computers that dominate trading have become too complex to control. Special curbs adopted after the May 2010 equity crash helped calm today's volatility.

"All of a sudden, there was choppy trading and some stocks were halted," Arthur Hogan, a strategist at Lazard Capital Markets LLC, said in a telephone interview. "People were scratching their heads, but it wasn't a sense of panic. It was more curious. There's got to be some human error here."

Goodyear Tire & Rubber Co. rose more than 10 percent just after the 9:30 a.m. open in New York. Manitowoc Co. gained 14 percent, Pandora Media Inc. climbed almost 11 percent and Level 3 Communications Inc. plunged 15 percent before the swings narrowed minutes later, according to data compiled by Bloomberg.

Internal Review

"An initial review by Knight indicates that a technology issue occurred in the company's market-making unit related to the routing of shares of approximately 150 stocks to the NYSE," according to a Knight e-mail from spokeswoman Kara Fitzsimmons.

"Knight notified its market-making clients this morning to route listed orders away," according to the statement. "The company's OTC securities and trading in its other businesses are not affected. The company continues to review internally."

Knight's market-making business traded a daily average of $19.5 billion worth of equities in June with volume of 3.1 billion shares, according to the company's website.

The New York Stock Exchange said it was reviewing trades in 148 securities between 9:30 a.m. and 10:15 a.m. New York time, according to a statement on its website.

"At this time, we believe NYSE systems and circuit breakers operated normally during this period, and we are working with all market participants on the issue," NYSE said in an e-mailed statement.