When computerized stock trading runs amok, as it did this week on Wall Street, the firm responsible typically can jump in and hit a kill switch.
But as a torrent of faulty trades spewed Wednesday morning from a Knight Capital Group trading program, no one at the firm managed to stop it for more than a half-hour.
Some Knight employees and New York Stock Exchange officials noticed the blizzard of erratic orders in the minutes after trading started and sent alarmed messages to Knight managers, according to the exchange and Knight employees who declined to be identified discussing the matter.
As Knight struggled to survive on Friday, employees at the company, market overseers and other electronic trading firms were asking the same basic question: Where was the off switch?
Several market insiders said that they were bewildered, because in a market where trading losses can pile up in seconds, executives typically have a simple command that can immediately halt trading.
"Even just a minute or two would have been surprising to me. On these time scales, that is an eternity," said David Lauer, a trader at a high-speed firm until a year ago. "To have something going on for 30 minutes is shocking."
Regulators are planning to look into why there was such a lag between the discovery of the problem and when Knight's trading ceased, according to people with knowledge of the discussions. But so far the company has not provided any answers, even to its own staff, employees said.
On Friday, Knight, which in the last decade grew into a leading broker for American stocks, climbed off the mat, securing emergency financing that allowed it to continue operating for the day. It also enticed some of its customers to resume sending client stock trades, two days after it disclosed a possibly fatal $440 million loss from the software problem. But it faced a desperate weekend of maneuvering to find a more permanent solution for its woes. Knight's short-term financing was meant to keep it alive until Monday, when its executives and advisers hope to have deals completed to remove any doubt about the firm's future.
Advisers, including Sandler O'Neill & Partners, have been talking with Knight rivals and private equity shops about either buying divisions of the firm or investing in the business.
Among the businesses that Knight is in discussions about selling is its futures brokerage unit, largely made up of operations the firm purchased only in May, according to people briefed on the matter. Potential buyers for the business include R. J. O'Brien, which is based in Chicago and is one of the oldest futures clearing firms in the country.
Others that have expressed interest in potential investments or deals include rivals to Knight like the Citadel Investment Group, Virtu Financial and Peak6 Investments, as well as private equity firms like Kohlberg Kravis Roberts and TPG Capital, these people said.
Knight is also working with Goldman Sachs to help unwind the trades behind its extensive loss, according to people briefed on the matter.
Goldman has agreed to buy, at a discount, the shares that the trading firm had accumulated. Such a move would help Knight by taking the portfolio off its hands and freeing up capital.
Two major brokerage firms, TD Ameritrade and Scottrade, said on Friday that they had begun sending client orders to Knight. Others, like E*Trade Financial, had yet to resume doing so.
"Knight is one of many order routing destinations for us and has long been a good and trusted partner," Fred Tomczyk, TD Ameritrade's chief executive, said in a statement.
Toward the end of the trading day Friday, employees in the Jersey City offices gathered around TV screens and cheered at every bit of good news.
Shares in Knight leapt 57 percent on Friday, closing at $4.05. But they remained down more than 60 percent for the week.
Coming after a number of previous market mishaps caused by faulty computerized trading, Knight's trading problems rekindled a broader discussion about the vulnerability of an increasingly complex and fragmented stock market.
In a statement, the chairwoman of the Securities and Exchange Commission, Mary L. Schapiro, called the Wednesday episode unacceptable and said that her staff would "convene a round table in the coming weeks to discuss further steps that can be taken to address these critical issues."
Duncan Niederauer, the chief executive of the New York Stock Exchange, said in a conference call with investors that the incident was a "call to action," and that the exchange was prepared to lead the way on reforms.
"We are all understanding - meaning we, market participants, and most importantly the regulators - are understanding that speed is not always better," Mr. Niederauer said.
Within the financial community, much of the attention was still focused on what happened Wednesday morning.
While the New York Stock Exchange has said that there was "irregular trading" in only about 140 stocks listed on its exchange that day, Knight's trading in those stocks was so extreme that it was visible in the volume of trading in all stocks.
A New York Times analysis of New York Stock Exchange volume on Wednesday morning showed that during the first minute of trading there was 12 percent more trading in all stocks than there had been on average during the previous seven days. By the third minute of trading there was 116 percent more trading than the previous week's average. The difference reached a peak at 9:58 a.m., when the volume was six times greater. After that, trading volume fell off sharply, nearing the recent average at 10:15 a.m.
Mr. Niederauer said that the exchange had noticed the problem and contacted Knight "within minutes" of the 9:30 opening bell.
Knight's failure to respond sooner was particularly mystifying to other traders because on Wednesday the firm had introduced new trading software. Industry experts said that this would normally be cause for programmers and other employees to be on high alert.
Once the problems began, many traders said it would have made sense if the firm's employees had not caught the problems for the first minute or so, given the speed at which Knight's program was firing off orders. After that, though, the problems were visible for all to see.
Howard Tai, an expert in high-speed trading at the Aite Group, said that at all the firms where he worked, there were several warning signals built into every computerized trading system. When all else failed, there was always the "automatic kill switch" that could immediately stop trading.
Mr. Lauer said, "It's kind of mind-boggling that it got so out of control."
Azam Ahmed and Ben Protess contributed reporting.