NEW YORK— As computer errors at one of the largest U.S. market makers sent stocks swinging as much as 151 percent, Joseph Cangemi, a broker at ConvergEx Group, acted like a driver approaching a car crash. He slowed down.

“You have to put on your extra-sensitivity helmets and look for every possible bit of information to make prudent decisions,” said Cangemi, the head of equity sales and trading at the New York-based firm. “We were in a heightened state of alarm, and we’re actually still in a heightened state of alarm.”

The New York Stock Exchange reviewed trading in 140 stocks from Molycorp to AT&T on Wednesday as the market’s open was rocked. Shares of Knight Capital Group, whose algorithms help execute about $20 billion of trades on an average day, tumbled the most ever on speculation it was responsible. Trades that occurred during the height of the volatility were canceled in six securities.

While almost all stock transactions in the United States used to be handled by three exchanges, regulations to increase competition and reduce costs have fragmented markets across about 50 different venues, raising concerns about integrity when the computers that increasingly dominate trading malfunction. For some investors, Wednesday’s disruptions were a reminder that the issues raised in May 2010 when the so-called flash crash briefly wiped out $862 billion from U.S. equities have yet to be solved.

“Everyone’s scratching their heads and going, ‘OK, well, who spilled coffee on the computer again?’ “ Peter Sorrentino, who helps oversee $14.7 billion as a senior money manager at Huntington Asset Advisors in Cincinnati, said in a phone interview. “After the flash crash, any time you see something like this, you definitely pause.”

Wednesday’s swings were caused by a malfunction in a so-called trading algorithm, according to a person at Knight who asked to remain anonymous because the matter hasn’t been publicized. The Securities and Exchange Commission and Commodity Futures Trading Commission blamed a broker’s algorithm for setting into motion the events that caused the market crash in May 2010.

Among the stocks that were reviewed by the NYSE were nine members of the Dow Jones Industrial Average, companies with almost $1 trillion in combined market value. They included Alcoa, American Express, Bank of America and AT&T.

Instant messaging systems used by traders were full of messages saying “What the heck, what’s going on, there’s no news, I’m not aware of any news, dig into this, has anybody heard anything?” said Brian Barish, who oversees $7 billion as president and chief investment officer at Denver-based Cambiar Investors.