The queasy chaos of this week’s markets, which has rattled even Wall Street pros, appeared to hit smaller investors especially hard, leaving a fresh dent in their stock market confidence.

Millions of these Main Street investors were locked out during the crucial hour when the worst hit, just as markets opened Monday. Popular trading platforms run by TD Ameritrade, Scottrade and others ran slow or not at all as panic grabbed hold. It took just six minutes for the Dow Jones industrial average to suffer its biggest drop in history. And these investors could only watch.

“It makes me wonder if a guy like me has a fair chance or not,” said Israel Hernandez, a lawyer in Casa Grande, Ariz., who could not log onto his online broker.

In a blink, mayhem descended. Strange glitches emerged. Stocks fell like rocks, only to shoot back up minutes later. Exchanges spit out the wrong prices for widely held funds. These problems are now being fingered as a potential reason many investors could not trade. Some experts are now calling it a flash crash, harking to May 2010 when stocks, largely because of technical problems, instantly plunged for a moment and then recovered, an incident that spurred new market rules.

Now the hunt for answers is underway, with some professional traders and market analysts saying the technology that has made the markets such a modern marvel is making it harder for everyone but a select few investors, raising questions about whether the exchanges offer a fair playing field. While folks such as Hernandez were locked out, some Wall Street players boasted of making huge profits amid the chaos.

The Securities and Exchange Commission, which has pledged repeatedly to shore up the increasingly complex systems for buying and selling stocks, declined to address particulars, saying it was “monitoring market developments.”

No one seemed prepared for the events unfurling from the markets’ opening Monday. Blue-chips such as General Electric experienced odd 8 percent swings inside of a second. Popular exchange-traded funds bounced like rabid yo-yos. Trading circuit breakers — forced time-outs — blew 100 times more often than normal.

In an hour, it was mostly over. Markets have remained shaky this week, but they at least seem to be making sense again, reacting to news such as the economic storm clouds surrounding China.

But the manic action Monday seemed most unnerving for investors who lack the access or credentials to play the markets at the highest level.

One reason was that the types of securities worst-hit by the day’s confusing gyrations — exchange-traded funds, or ETFs — are the same securities that financial advisers and TV ads have pushed as good vehicles for Main Street investors.

ETFs have gained traction in recent years, reaching $2.1 trillion in investments in the United States. Marketed as low-fee alternatives to mutual funds, ETFs track a select basket of stocks. Investors can buy ETFs in almost any category, such as the Standard & Poor’s 500-stock index or the health sector, allowing them to follow the ups and downs in a variety of stocks without buying each one individually.

Most ETFs are mundane, unexotic tools. But at least a dozen popular ETFs dropped off a cliff without explanation at 9:30 a.m. Monday. The Vanguard Consumer Staples fund was briefly down 32 percent. The iShares Select Dividend fell a similar amount.

In 15 minutes, the SPDR S&P Dividend ETF — heavily sold via TV ads — dropped 33 percent. It shot right back up 30 minutes later. And the stocks tracked by the ETF never fell that far.

It was like nothing happened — except for investors who had automatic “stop-loss” orders triggered by the brief fall. Those investors locked in heavy losses. “The retail guy got crushed,” sid Joe Saluzzi, partner at Themis Trading, a brokerage firm. “That shouldn’t be allowed.”

The fall and the bounce were inexplicable. It was like a mini flash crash of its own.

The fate of another ETF captured the market craziness, said Eric Scott Hunsader, chief executive of Nanex, market data provider to clients such as hedge funds.

PowerShares QQQ tracks 100 Nasdaq stocks. It is one of the most traded ETFs. And it quickly dived 17 percent.

“This wasn’t retail doing this,” Hunsader said, referring to Main Street traders. He said he believes the plunge was driven by computer algorithms feeding off the turbulence and amplifying it, all in split-second increments.

The shares of the 100 companies that make up the PowerShares QQQ did not drop 17 percent. A similar break between an ETF and its stock basket was seen with the Vanguard Consumer Staples ETF; its 32 percent drop came as the underlying securities were down only 9 percent. That massive gap was an incredible profitmaking opportunity for someone, most likely a major player with the fastest computers and best algorithms.

All ETFs eventually resumed normal pricing Monday. But these funds never should act like that, Hunsader said: “Hundreds of them completely disintegrated. And that’s quite troubling.”

Many traders pointed to a liquidity problem — the morning’s gyrations scared off brokers who normally serve as market makers connecting buyers and sellers. These market makers could not get accurate prices.

Matt Hougan, chief executive of, which follows the industry, said that he is sympathetic to the market makers but that they left the funds in a perilous position. “This doesn’t give anyone confidence in ETF trading,” Hougan said. “I think regulators should look at it.”

Hunsader pointed to the high-frequency traders as the problem. Normally, these firms make money by employing computers to make trades off micro-movements in stocks over very short bursts of time. But on Monday, many stopped buying and selling when they were needed most, he said. The only thing that prevented a complete market collapse was the circuit breakers that temporarily halted trading on various stocks, he said.

Bill Harts of Modern Markets Initiative, a high-frequency-trading advocacy group, said his clients worked hard to clear orders, especially sellers whose orders accumulated during the forced trading time-outs. “HFTs acted as responsible intermediaries, providing liquidity throughout the tumultuous trading on Monday,” he said.

It will probably take months to sort out the causes behind Monday’s rapid meltdown.

Mom-and-pop investors should not be tracking the markets moment by moment anyhow, said Mercer Bullard, a University of Mississippi law professor focused on securities.

But even just cursory glances at how stocks performed, especially during the wild morning hours, did nothing to reassure. Bullard said extreme volatility is ideal for major Wall Street players. “There is no question, to a wide extent, the markets are fixed,” Bullard said.

Hernandez, the investor stuck on the sidelines, said he wanted only to have the same opportunity to snatch up the bizarre bargains created by Monday’s unruly conditions. He couldn’t.

His broker, TD Ameritrade, said trading problems were reported on platforms industry-wide on a day when it recorded all-time highs in activity among its 6.6 million accounts. “We recognize the day was not perfect,” spokeswoman Kim Hillyer said.

Hernandez got five commission-free trades as compensation.

But, he said, he wasn’t sure when he was going to trade again.