The Federal Bureau of Investigations is looking into the practices of high-frequency stock trading firms, adding to the scrutiny that is being placed on the practice.
Brokerage firms use high-frequency trading to get a jump on their competitors. Powerful computers analyse market information and then execute buy and sell orders for stocks within a fraction of a second.
One way that high-frequency traders have gained an edge is by receiving market-moving information, such as corporate earnings releases, before other traders and investors.
They then exploit that advantage by placing buy or sell orders before other investors.
Until recently, the firms have been able to access crucial financial information by subscribing directly to companies that publish corporate reports, rather than accessing it through financial news wires such as ThomsonReuters, Dow Jones and Bloomberg.
That particular practice has come under increasing scrutiny in recent months, though.
New York attorney general Eric Schneiderman says high-frequency trading gives firms an unfair advantage and erodes public confidence in the stock market.
Business Wire, a company that distributes corporate earnings and other news releases, in February said it would stop providing its service directly to high-frequency trading firms.
Even though the direct distribution of its electronic feeds to a "handful" of trading firms was not illegal, the company said it was concerned about its reputation.
The move was followed last month by Marketwired, a company which provides a similar service.
The FBI has been investigating high-frequency trading for about a year, according to a person familiar with the matter, who spoke on condition of anonymity.
The Wall Street Journal reported on Tuesday that investigators were examining the practice of placing a group of trades and then cancelling them to create the false appearance of market activity.