The financial crisis of 2008-09 had several causes, among them complex financial instruments that few investors understood.

Wall Street’s relentless pursuit of ingenious new ways to make money and the failure of regulators to see increasing risks in the financial markets were also factors in the crisis.

Today, Wall Street has again found ways to make money that few people understand. It’s called high-frequency trading — HFT — and most people have never heard of it. And of those who have heard of it, few understand it.

As the name implies, it’s all about speed — and the traders with the fastest fiber optic cables and computers win. The high-frequency traders buy and sell millions of shares a day, skimming a fraction of a cent from most transactions. They end the day holding no stock. They do not invest; rather, they are traders, interjecting themselves between buyers and sellers, making money in the process.

Regulators and other federal officials have been watching the high-frequency traders and are paying attention. Investigations have been started by the Securities Exchange Commission, the Federal Bureau of Investigation and the U.S. Justice Department. New York States’ attorney general has called this high-tech, high-speed trading edge “Insider Trading 2.0.”

The high-speed traders might not be breaking the law, but they are gaming the system, placing themselves between buyers and sellers of stock and using their super high-speed connections and ultra-fast computer programs to act on advance information. Critics have another name for HFT: “Front running.”

The HFT’s speed advantage lets them know buy and sell offers for large blocks of companies’ stock a few millionths of a second before ordinary traders or brokers. The impact of HFT on average investors is very small. The high-speed traders shave off or skim a fraction of a cent per share as they inject themselves between normal buyers and sellers. But with hundreds of millions of shares traded every day, these virtually invisible stock traders make billions of dollars a year.

The increasing influence of high-speed traders is creating several problems. It’s not illegal, but is a way of gaming the system and the unfairness of their methods is attracting attention.

The high-speed traders are now responsible for more than 60 percent of total stock market transactions and their computer-driven lightning-fast buying and selling is believed to have been a factor in the so-called “flash crash” of 2010, when the stock market suddenly dropped 500 points, only to recover a few minutes later. So, the apparent link between high-frequency trading and market volatility is another concern.

While volatility adds risks for regular investors, it looks like gold to high-speed traders who take advantage of tiny differences between offers to buy or sell a stock. The more volatility in the stock market, the more opportunities for high-speed traders to skim money from every transaction.

In the past few years, more people have been learning about the growing role of high-speed traders and how they are gaming the system. There have been stories of these traders paying huge sums to locate their trading computers in the building right next to the traditional stock exchanges, so the distance that their electronic messages to buy or sell is shorter — by even a few hundred feet. The shorter distance buys the traders a few milliseconds, enough time to beat the other guys and skim a little bit off stock trades.

Investigations into high-speed trading and so-called dark pools that are like stock exchanges operating with little transparency are being launched and could lead to legal action. Even the increased scrutiny of the high-speed traders might lead to market reforms.

Beyond the legal and regulatory attention on HFT, a just-released book by Michael Lewis, titled “Flash Boys” tells the story of high-speed trading and what a small group of people with financial and computer coding expertise did to try to cut out those traders’ gaming of the system.

Complexity and lack of transparency were ingredients in the 2008 financial crisis. High-speed traders have been making fortunes for some of the same reasons, while gaming the system. The recent attention to HFT could change the rules, bringing more transparency and less a sense that the market is rigged, and high-frequency traders make millions without taking any risk.